Firebase auth returning immediately using Sign in with Apple

I have configured an App ID and a Services ID with my web app's domain name and auth callback URL for Firebase on the Apple Developer portal.

However, on calling the Firebase auth method:

async appleAuth() {
  if (this.auth) {
    const provider = new OAuthProvider('apple.com');
    const auth = firebase.getAuth();
    try {
      await signInWithPopup(auth, provider);
    } catch (error) {
      console.log(error);
    }
    return firstValueFrom(this.isLoggedIn!);
  }

  return false;
}

I receive the error:

FirebaseError: Firebase: Error (auth/operation-not-allowed).

Of course I have configured Apple ID sign in with Firebase.

What component might I be missing??

One fly in the ointment might be that I am using my own domain's callback URL, although Firebase tells me that I need to add theirs to Developer portal. I have done that too, but it might be interfering? I must use my own, otherwise Google won't work.