Instagram returning “Matching code was not found or was already used” when using OAuth

Question

I am trying to use the Instagram OAuth using the developer documentation at

https://www.instagram.com/developer/authentication/.

Step 1 and 2 a

Answer 1

I got the same error message. After 2 hours trying every workaround, I realized that I was using FB secret code instead of IG. Double check it first before trying any other solutions.