user20381: I've been using Arduino UNO for quite sometime and brought a new Arduino Nano. When I connected it to my computer, the power usb lit up and the blink sketch started running ( as expected of course).  
Now I opened up the IDE and switched boards. But when I try to upload code to my Nano I get an error- 

avrdude: ser_open(): can't open device "\.\COM3": The system cannot find the file specified.

Clearly the IDE can't see my board. Uploading to my UNO goes fine. The serial port option is greyed out so I can't change it.
I have searched extensively online for a solution and have found none. This must be either a very unique issue or a very simple one.

user4143: Firstly, you have a clone not a genuine Arduino Nano. They use a different USB chip and require different drivers. Install the CH340 drivers.
Secondly your cable is bad. It sounds like one with no data wires in it used for powering or charging small low-powered devices like bluetooth headsets etc. Replace the cable with a proper one.

user69494: I've seen this error a few times and renaming the serial port fixed the problem in my case. I documented the steps in detail in Tip #4 of this blog post: https://gyorgybalassy.wordpress.com/2020/10/16/cant-open-device-com-access-is-denied-arduino-upload/

user2830: When using a cheap knockoff Nano, it seems that in the newer versions of the Arduino IDE (at least in 1.8.8), you may need to select "ATmega328P (Old bootloader)" from the Processor options under the Tools menu in order to upload sketches.

20381: <p>I've been using Arduino UNO for quite sometime and brought a new Arduino Nano. When I connected it to my computer, the power usb lit up and the blink sketch started running ( as expected of course).  </p>

<p>Now I opened up the IDE and switched boards. But when I try to upload code to my Nano I get an error- </p>

<blockquote>
  <p>avrdude: ser_open(): can't open device "\.\COM3": The system cannot find the file specified.</p>
</blockquote>

<p>Clearly the IDE can't see my board. Uploading to my UNO goes fine. The serial port option is greyed out so I can't change it.</p>

<p>I have searched extensively online for a solution and have found none. This must be either a very unique issue or a very simple one.</p>
 4143: <p>Firstly, you have a clone not a genuine Arduino Nano. They use a different USB chip and require different drivers. Install the CH340 drivers.</p>

<p>Secondly your cable is bad. It sounds like one with no data wires in it used for powering or charging small low-powered devices like bluetooth headsets etc. Replace the cable with a proper one.</p>
 69494: <p>I've seen this error a few times and renaming the serial port fixed the problem in my case. I documented the steps in detail in Tip #4 of this blog post: <a href="https://gyorgybalassy.wordpress.com/2020/10/16/cant-open-device-com-access-is-denied-arduino-upload/" rel="nofollow noreferrer">https://gyorgybalassy.wordpress.com/2020/10/16/cant-open-device-com-access-is-denied-arduino-upload/</a></p>
 2830: <p>When using a cheap knockoff Nano, it seems that in the newer versions of the Arduino IDE (at least in 1.8.8), you may need to select "ATmega328P (Old bootloader)" from the Processor options under the Tools menu in order to upload sketches.</p>
