There seems to be some confusion.

The check in agent at the airport told me the system did not allow her to UG US Chairman Preferred members. When I told her they do in the USA she said the system in Mexico was not capable. I asked for availability and she said they had 7 seats open. That would be about 1 hrs before departure.

When I saw the kiosk it was the generic multi airline kind. I pushed AA and scanned my passport. It was clearly an AA app running. During the checkin process there was a button to buy up the UG. I picked it and it gave it to me for zero dollars.

I guess the system did not UG me instantly. I chose to buy the UG and it did not charge me for it as a USCP.

I guess if I were flying AA again I would try the kiosk again and do the buy up rather than wait for an agent. Of course YMMV.