com.google.firebase.FirebaseException: An internal error has occurred. [ CONFIGURATION_NOT_FOUND ]

Check if you enabled the authentication type you want in the firebase console.

Have you checked the project_id in your Android app google-services.json file is the same as the Firebase project for which you enabled Google Sign In Authentication? If that is the case and you have not edited the google-services.json file, you can file a support ticket to Firebase.

Enabling Sign-in method (email/password in my case) in Firebase project dashboard fixed the issue for me.