Android Studio：com.google.android.gms.auth.GoogleAuthException：BadUsername

I simply do not know what the issue is. I read all API docs and looked at 1 available StackOverflow question to no avail. I did generate the client ID and stuff that match my package name and debug SHA1 code. My goal is to connect to a specific Google account regardless of user and the device. In a sense that account is a back-end database for me but functions as regular drive account. I do not want anything to do with AccountManager/Picker either. This seems to be such shady and obscure subject and I would greatly appreciate if anyone shed some light on it.I simply do not know what the issue is. I read