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 localhost
.
Let me know if it still doesn't work, I'll try to help as I can, if you provide more information. Good luck!