I just created a new Apple ID for my dad but my AppleTV won't add it

So I created a new Apple ID for my dad because basically I don't want to mix up our libraries and play history and all that stuff. Anyway, when I try to add the new ID, I keep getting and error saying "Failed to Add User" and "There was a problem adding the new user." Is there a reason why it's doing this? I'm kind of getting annoyed. I tried resetting the device as I saw recommended somewhere but that didn't work. Any suggestions?