Sorry for the super late reply!
I've created a repo with sample code here: https://github.com/JagCesar/sign-in-with-apple-example
When trying to sign in on iOS it works correctly,
func authorizationController(controller: ASAuthorizationController, didCompleteWithAuthorization authorization: ASAuthorization)
is called as expected.
When trying to sign in on a Mac Catalyst build of the same code doesn't work though, instead I get this error in the "Sign in overlay".
https://github.com/JagCesar/sign-in-with-apple-example/raw/master/mac%20error.png
Not sure if this is a bug or a feature but if it is a feature it's sadly blocking all Catalyst apps from using Sign in with Apple which would be sad 😟 I was looking forward to making all my users anonymous!