The American Airlines website requires me to enter the country for my Known Traveler Number (Global Entry/PreCheck) in order to sell me a ticket. It doesn’t remember the country (United States!) after I’ve entered it.
The website does not give me a field in which to add this for any passenger besides myself when buying a ticket for my family and entering their AAdvantage number. That means every time I try to buy a ticket the website throws an error.