com.firebase.ui.auth.FirebaseUiException: Code: 16, message: 16:

This error is not yet mapped to a developer-friendly message. To figure out the actual problem, I spent around 2 hours for this. And it turned out to be a silly mistake.

The Solution:

Your signing certificate SHA-1 doesn’t match your Firebase project setup. 

After updating the correct SHA-1 fingerprint of the keystore, it started working again.

This happened to me because I was switching from 1 dev machine to another, and I forgot to add the new certificate fingerprint. I hope, in the future, the SDK should provide a better error message instead of an error code.