Question: Sign In with Apple returns invalid_grant
Answer:
This is often caused by a mismatch of the Bundle ID. Make sure the Bundle ID is the same in your native app, and the Application configuration in the Auth0 dashboard, under “Advanced Settings → Device Setting → App ID”.
Apple also returns invalid_grant when the authorization code has been already consumed. Make sure your app is not sending a code that was already exchanged for an access_token.
Supporting Documentation:
Documentation: Add Sign In with Apple to Native iOS Apps