Hi, I don't really recommend logging in to a native mobile app (Android or iOS) using web app authentication of Firebase, like what you're using. It has been long depreciated by Google and Apple. There is no guarantee that it will work anymore.
You should use native authentication, like what we use for PlayFab + Mobile Facebook or Mobile GooglePlus.
But, if you have your reasons, try this:
Usually this happens when there is only a missing app configuration from the Firebase Console, try this, skip the code parts. And, don't forget to whitelist the
Let me know if it still doesn't work, I'll try to help as I can, if you provide more information. Good luck!