If you haven't yet specified your app's SHA fingerprint, do so from the Settings page of the Firebase console. Refer to Authenticating Your Client for details on how to get your app's SHA fingerprint.
Go to the Authentication section > Sign-in method > Add new provider > Google
When prompted in the console, download the updated Firebase config file (google-services.json), which now contains the OAuth client information required for Google sign-in.
Move this updated config file into your Android Studio project, replacing the now-outdated corresponding config file.