Hi @er31, welcome to our forum!
This is because of the anti theft feature called Google Device Protection and after a reset, you will need to log in with the Google account that was present in the device prior to the reset (unless it was removed and a factory reset was made).
The only way around this protection if you can't get a hold of the original owner is to have the device sent in to an authorized service centre, together with a copy of the receipt. The service centre can then remove the protection on the device and once you add your own Google account, this is the account that the device will be "locked to".
For further assistance with having the device sent in, get in touch with your Local support team.
I hope that helps!
