In order to verify your amazon account you need to submit your amazon profile link, which is under your account→ see all account → your profile. Also this is only possible in a web browser not the app.
This link will take you directly to your account screen (if you are not logged in please do that first in your browser).
Copy the text mentioned on the OPA app and add that text in your amazon bio, this text is to verify whether the amazon account belongs to you or not.
Once you have updated your profile with the bio text, copy the bio by clicking on the copy icon of your amazon account.
Submit the copied URL on the OPA app, it should verify your account.