I have a 2 year old iphone 6. I can't add my fingerprint (the option is gray) and it always say "unable to comple touch id setup" Tried reset, rebooting, etc and no change at all. The battery became easily drained suddenly. PLEASE SOMEBODY HELP
Sounds like more of a hardware issue you might want to get checked out by Apple. Only other thing I can think of is if you wiped and restored the phone as new without restoring from back up to see if there is a deeper bug that hard resets won't fix.