You can absolutely buy your AP at the Universal Ticket Desk in the hotel (not check in, it is a separate desk) every Universal Hotel has them in the lobby. Just wait and buy them there, not online beforehand.
If you purchased a “special promotion” online and need to make a complicated upgrade, make changes, change a name, someone else paid for them as a gift, etc….etc….etc….
Then you may have to visit guest services at the park.
I have used the AP rate and then purchased our AP’s at the hotel several times in 3 of the Hotels, RPR, SF and CB.