Can't sign in on an Android phone with a Google account


Deleted User

My brother reset his phone and now I can't sign in.
Google requires me to enter an account which was previously synced with the device.
Unfortunately, my brother doesn't remember the account he used to sign in on his phone.
I reset the password to two of his accounts, but, without any luck. Not even a single one of them was used on the device before.
I get this "Verify your account" screen and when I enter any Google account, it won't let me in.
So, the phone is useless.
Following YouTube tutorials, I tried to bypass this "Verify your account" thing by resetting the phone from Settings. Again, without any luck.

The device is not stolen or something like that.
I can't find the account that was previously synced with this device.
I don't want to root the phone or do stuff like that, because I will lose the guarantee of my service provider.

What should I do?
Thank you!