"Invalid Amazon credentials" error

1) First of all, make sure you have properly generated your 
Secret Access Key and Access Key ID fields.

2) Make sure you don't mismatch your Secret Access Key and Access Key ID fields.

3) Make sure your CAPS button isn't on.

4) Make sure you don't accidentally copy and past an extra space in the beginning or the end of the characters.

Try adding the characters manually, one by one.

You can also wait for 24 hours and see if the issue goes away on its own?

If the issue persists, please contact Amazon Associate support.