Are you using the Official Windows 10 upgrade from AT&T or are you on preview upgrade? Is your OS version 10.0.10586.420? If you are not on the Official Windows 10 upgrade, then I would suggest that you roll back your phone to WP8.1 first. Then download the Upgrade Advisor app from the Store and run that program to see if your phone can be updated.
I did not have to do a hard reset for my L1520 for Windows 10 OS to work.