title: MySQL - Can't create second foreign key
body: <p>I'm trying to create second foreign key in my table 'tracker_clicks' -
  <a href="https://i.stack.imgur.com/usogg.png" rel="nofollow noreferrer">image1</a></p>
  
  <p>The first foreign key is for column 'campaign_id' and it has been created successfully.</p>
  
  <p>But the second one is creating for 'offer_id' column to 'tracker_offers' table - <a href="https://i.stack.imgur.com/JCFCo.png" rel="nofollow noreferrer">image2</a></p>
  
  <p>And I get this error - <a href="https://i.stack.imgur.com/YTadD.png" rel="nofollow noreferrer">image3</a></p>
  
  <p>What's wrong?</p>