Regardless of the way UA handles point of sale restrictions (by billing country, rather than country of departure lIke most, and only allows use of a card that matches country of the website), that’s not going to be the only issue here. If this truly is an AC (014) ticket, I don’t think you could actually buy an upgrade. I have no experience in trying, but I believe an upgrade would require a ticket re-issue due to change of cabin. UA can’t do this if AC has control of the ticket - UA wouldn’t have control of the ticket to do this until day of the UA segment, at the airport. E+ shouldn’t require this since cabin remains the same, it’s only the seat selection that changes (though some carriers supposedly can’t handle paid seat selection on another carrier ticket).

If this was a UA ticket, by the way, calling and doing this with an agent should solve the issue - they should be able to process a payment on any card, and can upgrade you at the same price as online.