Firebase Android: An internal error has occurred. [ OPERATION_NOT_ALLOWED ]

I had the same problem. I got the solution in the forum post [Google Auth] com.google.firebase.FirebaseException: An internal error has occurred.

This happened to me when I did not have the authentication method enabled in the console Firebase. When I enabled Google authentication, I got the same exception, just without [OPERATION_NOT_ALLOWED].

If you are signup with email and password then follow below steps.

Go to https://console.firebase.google.com/
Select your project.
Click on Authentication from menu option(Right-side menu)
Click on SIGN-IN-METHOD
Click on Email/Password and enable it.
Click on SAVE.

1. Go to https://console.firebase.google.com/
2. Select your project.
3. Click on Authentication from menu option
4. Click on SIGN-IN-METHOD
5. Click on Google and enable it.

Than it works fine :)