None of the answers to this question have even vaguely answered it but have gone off on tangents instead. It has nothing to do with the airline, whether it is accepted or not depends upon the airport, not the airline. But that is besides the point, the question is how do you physically add a boarding pass to Apple Wallet. American Airlines sends an email to you which contains a link to your boarding pass. American further advises you to add the pass, that appears after you have followed that link, to your Apple Wallet. The procedure should be to open Wallet, click on the + sign in the top right corner and then position the boarding pass within the box in Wallet until Wallet clicks and records the boarding pass. Unfortunately, nothing seems to happen. You never get to that "click" and the question is why? I agree with the person who posted this question earlier: it should be easy, so why isn't it?
After you make the check-in online (either via the app or the website), the American Airlines app allows you to add the boarding pass to you wallet from inside the app (it's in effect the last step of the check-in).
But if you have a "generic" boarding pass you can try with this site: