user33872: I am getting this error.
Arduino: 1.8.2 Hourly Build 2017/02/17 11:33 (Windows 8.1), Board: "Arduino/Genuino Uno"

Tone.cpp.o (symbol from plugin): In function `timer0_pin_port':

(.text+0x0): multiple definition of `__vector_7'

libraries\REMOTE\IRremote.cpp.o (symbol from plugin):(.text+0x0): first defined here

collect2.exe: error: ld returned 1 exit status

exit status 1
Error compiling for board Arduino/Genuino Uno.

This report would have more information with
"Show verbose output during compilation"
option enabled in File -> Preferences. 

Here is the entire code: 
#include <RFID.h>
#include <Wire.h>  // i2C Conection Library
#include <LiquidCrystal_I2C.h>  //Memanggil i2C LCD Library
LiquidCrystal_I2C lcd(0x3F, 16, 2);

#include <SPI.h>
#include <RFID.h>
#define SS_PIN 10
#define RST_PIN 9

RFID rfid(SS_PIN,RST_PIN);

const int buzzer = 7;   //Passive buzzer ke Pin D7 dan GND
const int doorLock = 8; //Driver Selenoid Door Lock/Relay input ke pin D8
int serNum[5];          //Variable buffer Scan Card

int cards[][5] = {      //ID Kartu yang diperbolehkan masuk
  {120, 181, 196 ,101, 14}

};

bool access = false;

int alarm = 0;
uint8_t alarmStat = 0;
uint8_t maxError = 5;
void setup(){
    Serial.begin(9600);
    lcd.begin();
    SPI.begin();
    rfid.init();

    pinMode(doorLock, OUTPUT);
    digitalWrite(doorLock, HIGH);

  tone (buzzer,1200);
  lcd.setCursor (0,0);
  lcd.print(F("  RFID System   "));
  lcd.setCursor (0,1);
  lcd.print(F(" Control Access "));
  delay (2000);
  lcd.clear();
  noTone (buzzer);
}

void loop(){
  if (alarm >= maxError){
    alarmStat = 1;    }

  if (alarmStat == 0){
  lcd.setCursor (0,0);
  lcd.print(F(" -System Ready- "));
  lcd.setCursor (0,1);
  lcd.print(F(" Scan Your Card "));

    if(rfid.isCard()){

        if(rfid.readCardSerial()){
            Serial.print(rfid.serNum[0]);
            Serial.print(" ");
            Serial.print(rfid.serNum[1]);
            Serial.print(" ");
            Serial.print(rfid.serNum[2]);
            Serial.print(" ");
            Serial.print(rfid.serNum[3]);
            Serial.print(" ");
            Serial.print(rfid.serNum[4]);
            Serial.println("");

            for(int x = 0; x < sizeof(cards); x++){
              for(int i = 0; i < sizeof(rfid.serNum); i++ ){
                  if(rfid.serNum[i] != cards[x][i]) {
                      access = false;
                      break;
                  } else {
                      access = true;
                  }
              }
              if(access) break;
            }   
        }

       if(access){
          Serial.println("Welcome!");
          lcd.setCursor (0,0);
          lcd.print(F(" DOOR UNLOCKED!! :)"));
          lcd.setCursor (0,1);
          lcd.print("ID:");
          lcd.print(rfid.serNum[0]); lcd.print(rfid.serNum[1]);
          lcd.print(rfid.serNum[2]); lcd.print(rfid.serNum[3]);
          lcd.print(rfid.serNum[4]);
           digitalWrite(doorLock, LOW);
           tone (buzzer,900);
           delay(100);
           tone (buzzer,1200);
           delay(100);
           tone (buzzer,1800);
           delay(200);
           noTone(buzzer);  
           delay(600);
           lcd.setCursor (0,0);
           lcd.print(F("WELCOME"));
           lcd.setCursor (0,1);
           lcd.print(F("AutoLock after "));
           for(int i=5; i>0; i--){
            lcd.setCursor (15,1); lcd.print(i);
            delay (1000);
           }
           digitalWrite(doorLock, HIGH);
           lcd.clear();
      } else {
           alarm = alarm+1;
           Serial.println("Not allowed!"); 
           lcd.setCursor (0,0);
           lcd.print(F(" ACCESS DENIED!!  "));
           lcd.setCursor (0,1);
         lcd.print("ID:");
           lcd.print(rfid.serNum[0]); lcd.print(rfid.serNum[1]);
           lcd.print(rfid.serNum[2]); lcd.print(rfid.serNum[3]);
           lcd.print(rfid.serNum[4]);
           tone (buzzer,900);
           delay(200);
           noTone(buzzer);
           delay(200);
           tone (buzzer,900);
           delay(200);
           noTone (buzzer);  
           delay(500);
           lcd.clear();      
       }        
    }
    rfid.halt();
}
else {
  lcd.setCursor (0,0);
  lcd.print(F("-System LOCKED- "));
  lcd.setCursor (0,1);
  lcd.print(F(" Please Wait "));
for(int i=5; i>0; i--){
tone (buzzer,4000);

lcd.setCursor (13,1); lcd.print(i);
lcd.print(F("  "));delay (1000);}
noTone (buzzer);
alarmStat = 0;
alarm = 0;
  }
}


//code  of an IR REMOTE

#include <IRremote.h>

int RECV_PIN = 2; // the pin where you connect the output pin of TSOP4838
int lock =13 ; 
int itsONled[] = {};
/* the initial state of LEDs is OFF (zero) 
the first zero must remain zero but you can 
change the others to 1's if you want a certain
led to light when the board is powered */
#define code1 26775 // code received from button 1

IRrecv irrecv(RECV_PIN);

decode_results results;

void remote()
{
  Serial.begin(9600);   // you can comment this line
  irrecv.enableIRIn();  // Start the receiver0
  pinMode(lock, OUTPUT);

}

void loop_1() {
  if (irrecv.decode(&results)) {
    unsigned int value = results.value;
    switch(value) {
       case code1:
         if(itsONled[1] == 1) {        // if first led is on then
            digitalWrite(lock, LOW);   // turn it off when button is pressed
            itsONled[1] = 0;           // and set its state as off
            Serial.println("ledl OFF");
         } else {                      // else if first led is off
             digitalWrite(lock, HIGH); // turn it on when the button is pressed
            Serial.println("led 1 ON");
             itsONled[1] = 1;          // and set its state as on
         }

          break; 
    }                 

    Serial.println(value); // you can comment this line
    irrecv.resume(); // Receive the next value
  }
}


user3917: The error message multiple definition of __vector_7 means that you have more than one interrupt handler defined for the interrupt that has interrupt vector 7.
Although the ATmega328x datasheet in a table called Interrupt Vectors in ATmega328 and ATmega328P refers to interrupt vector 7 as the Watchdog Time-out Interrupt, that probably is misleading, because only the following ISRs can be defined in Tone.cpp:
ISR(TIMER0_COMPA_vect)
ISR(TIMER1_COMPA_vect)
ISR(TIMER2_COMPA_vect)
ISR(TIMER3_COMPA_vect)
ISR(TIMER4_COMPA_vect)
ISR(TIMER5_COMPA_vect)

(These can be defined, but typically most are not.  All of them are conditional upon which MCU is selected.)
The article multiple definition of '__vector_7' at arduino.cc attributes the problem to the last of those ISRs, ISR(TIMER5_COMPA_vect).   Anyhow, in short, you might need to check what board or MCU you are compiling for, or may need to find a substitute for one of Tone or IRremote, the libraries that are both defining the ISR that has a conflict.
Edit 1: The article TimerFreeTone Library v1.5: Play tones without timers and therefore no conflicts at arduino.cc describes a TimerFreeTone v1.5 library that generates tones without using timers.  You might be able to use that library in place of the Tone library that has a conflict.

33872: <p>I am getting this error.</p>

<pre><code>Arduino: 1.8.2 Hourly Build 2017/02/17 11:33 (Windows 8.1), Board: "Arduino/Genuino Uno"

Tone.cpp.o (symbol from plugin): In function `timer0_pin_port':

(.text+0x0): multiple definition of `__vector_7'

libraries\REMOTE\IRremote.cpp.o (symbol from plugin):(.text+0x0): first defined here

collect2.exe: error: ld returned 1 exit status

exit status 1
Error compiling for board Arduino/Genuino Uno.

This report would have more information with
"Show verbose output during compilation"
option enabled in File -&gt; Preferences. 
</code></pre>

<p>Here is the entire code: </p>

<pre><code>#include &lt;RFID.h&gt;
#include &lt;Wire.h&gt;  // i2C Conection Library
#include &lt;LiquidCrystal_I2C.h&gt;  //Memanggil i2C LCD Library
LiquidCrystal_I2C lcd(0x3F, 16, 2);

#include &lt;SPI.h&gt;
#include &lt;RFID.h&gt;
#define SS_PIN 10
#define RST_PIN 9

RFID rfid(SS_PIN,RST_PIN);

const int buzzer = 7;   //Passive buzzer ke Pin D7 dan GND
const int doorLock = 8; //Driver Selenoid Door Lock/Relay input ke pin D8
int serNum[5];          //Variable buffer Scan Card

int cards[][5] = {      //ID Kartu yang diperbolehkan masuk
  {120, 181, 196 ,101, 14}

};

bool access = false;

int alarm = 0;
uint8_t alarmStat = 0;
uint8_t maxError = 5;
void setup(){
    Serial.begin(9600);
    lcd.begin();
    SPI.begin();
    rfid.init();

    pinMode(doorLock, OUTPUT);
    digitalWrite(doorLock, HIGH);

  tone (buzzer,1200);
  lcd.setCursor (0,0);
  lcd.print(F("  RFID System   "));
  lcd.setCursor (0,1);
  lcd.print(F(" Control Access "));
  delay (2000);
  lcd.clear();
  noTone (buzzer);
}

void loop(){
  if (alarm &gt;= maxError){
    alarmStat = 1;    }

  if (alarmStat == 0){
  lcd.setCursor (0,0);
  lcd.print(F(" -System Ready- "));
  lcd.setCursor (0,1);
  lcd.print(F(" Scan Your Card "));

    if(rfid.isCard()){

        if(rfid.readCardSerial()){
            Serial.print(rfid.serNum[0]);
            Serial.print(" ");
            Serial.print(rfid.serNum[1]);
            Serial.print(" ");
            Serial.print(rfid.serNum[2]);
            Serial.print(" ");
            Serial.print(rfid.serNum[3]);
            Serial.print(" ");
            Serial.print(rfid.serNum[4]);
            Serial.println("");

            for(int x = 0; x &lt; sizeof(cards); x++){
              for(int i = 0; i &lt; sizeof(rfid.serNum); i++ ){
                  if(rfid.serNum[i] != cards[x][i]) {
                      access = false;
                      break;
                  } else {
                      access = true;
                  }
              }
              if(access) break;
            }   
        }

       if(access){
          Serial.println("Welcome!");
          lcd.setCursor (0,0);
          lcd.print(F(" DOOR UNLOCKED!! :)"));
          lcd.setCursor (0,1);
          lcd.print("ID:");
          lcd.print(rfid.serNum[0]); lcd.print(rfid.serNum[1]);
          lcd.print(rfid.serNum[2]); lcd.print(rfid.serNum[3]);
          lcd.print(rfid.serNum[4]);
           digitalWrite(doorLock, LOW);
           tone (buzzer,900);
           delay(100);
           tone (buzzer,1200);
           delay(100);
           tone (buzzer,1800);
           delay(200);
           noTone(buzzer);  
           delay(600);
           lcd.setCursor (0,0);
           lcd.print(F("WELCOME"));
           lcd.setCursor (0,1);
           lcd.print(F("AutoLock after "));
           for(int i=5; i&gt;0; i--){
            lcd.setCursor (15,1); lcd.print(i);
            delay (1000);
           }
           digitalWrite(doorLock, HIGH);
           lcd.clear();
      } else {
           alarm = alarm+1;
           Serial.println("Not allowed!"); 
           lcd.setCursor (0,0);
           lcd.print(F(" ACCESS DENIED!!  "));
           lcd.setCursor (0,1);
         lcd.print("ID:");
           lcd.print(rfid.serNum[0]); lcd.print(rfid.serNum[1]);
           lcd.print(rfid.serNum[2]); lcd.print(rfid.serNum[3]);
           lcd.print(rfid.serNum[4]);
           tone (buzzer,900);
           delay(200);
           noTone(buzzer);
           delay(200);
           tone (buzzer,900);
           delay(200);
           noTone (buzzer);  
           delay(500);
           lcd.clear();      
       }        
    }
    rfid.halt();
}
else {
  lcd.setCursor (0,0);
  lcd.print(F("-System LOCKED- "));
  lcd.setCursor (0,1);
  lcd.print(F(" Please Wait "));
for(int i=5; i&gt;0; i--){
tone (buzzer,4000);

lcd.setCursor (13,1); lcd.print(i);
lcd.print(F("  "));delay (1000);}
noTone (buzzer);
alarmStat = 0;
alarm = 0;
  }
}


//code  of an IR REMOTE

#include &lt;IRremote.h&gt;

int RECV_PIN = 2; // the pin where you connect the output pin of TSOP4838
int lock =13 ; 
int itsONled[] = {};
/* the initial state of LEDs is OFF (zero) 
the first zero must remain zero but you can 
change the others to 1's if you want a certain
led to light when the board is powered */
#define code1 26775 // code received from button 1

IRrecv irrecv(RECV_PIN);

decode_results results;

void remote()
{
  Serial.begin(9600);   // you can comment this line
  irrecv.enableIRIn();  // Start the receiver0
  pinMode(lock, OUTPUT);

}

void loop_1() {
  if (irrecv.decode(&amp;results)) {
    unsigned int value = results.value;
    switch(value) {
       case code1:
         if(itsONled[1] == 1) {        // if first led is on then
            digitalWrite(lock, LOW);   // turn it off when button is pressed
            itsONled[1] = 0;           // and set its state as off
            Serial.println("ledl OFF");
         } else {                      // else if first led is off
             digitalWrite(lock, HIGH); // turn it on when the button is pressed
            Serial.println("led 1 ON");
             itsONled[1] = 1;          // and set its state as on
         }

          break; 
    }                 

    Serial.println(value); // you can comment this line
    irrecv.resume(); // Receive the next value
  }
}
</code></pre>
 3917: <p>The error message <code>multiple definition of __vector_7</code> means that you have more than one interrupt handler defined for the interrupt that has interrupt vector 7.</p>

<p>Although the ATmega328x datasheet in a table called <em>Interrupt Vectors in ATmega328 and ATmega328P</em> refers to interrupt vector 7 as the Watchdog Time-out Interrupt, that probably is misleading, because only the following ISRs can be defined in <code>Tone.cpp</code>:</p>

<pre><code>ISR(TIMER0_COMPA_vect)
ISR(TIMER1_COMPA_vect)
ISR(TIMER2_COMPA_vect)
ISR(TIMER3_COMPA_vect)
ISR(TIMER4_COMPA_vect)
ISR(TIMER5_COMPA_vect)
</code></pre>

<p>(These can be defined, but typically most are not.  All of them are conditional upon which MCU is selected.)</p>

<p>The article <a href="http://forum.arduino.cc/index.php?topic=133553.0" rel="nofollow noreferrer"><em>multiple definition of '__vector_7'</em></a> at arduino.cc attributes the problem to the last of those ISRs, <code>ISR(TIMER5_COMPA_vect)</code>.   Anyhow, in short, you might need to check what board or MCU you are compiling for, or may need to find a substitute for one of <code>Tone</code> or <code>IRremote</code>, the libraries that are both defining the ISR that has a conflict.</p>

<p><em>Edit 1:</em> The article <a href="http://forum.arduino.cc/index.php?topic=235774.0" rel="nofollow noreferrer">TimerFreeTone Library v1.5: Play tones without timers and therefore no conflicts</a> at arduino.cc describes a <em>TimerFreeTone v1.5</em> library that generates tones without using timers.  You might be able to use that library in place of the <code>Tone</code> library that has a conflict.</p>
