I got this issue when setting an existing project on a new machine. Based on this StackOverflow post, what I needed to do was:

  • add my machine's Android Studio debug key to the list of whitelisted keys in Firebase's Android app.
  • download the updated google-services.json to android/app.

To get the SHA-1 key, you can either use keytool as explained in https://developers.google.com/android/guides/client-auth, or run ./gradlew signingReport. I did the latter.