I had a problem with Apple Sign-In not working in my Flutter app using Firebase on an iOS device (I got "Sign Up not completed" message on iPhone without any errors in logs).

Here’s what resolved it:

1. Updated macOS to the latest version (15.5).
2. Upgraded Xcode to the latest version (16.4).
3. (only for Flutter) Updated Flutter Firebase packages to their newest versions:
   firebase_core: ^3.14.0
   firebase_auth: ^5.6.0

After making these changes, Apple Sign-In started working correctly.

Additionally, I followed the steps outlined in this YouTube tutorial: https://www.youtube.com/watch?v=JEwGol44xFQ