I booked a trip on AC for Labor Day weekend and want to add my UA Mileage Plus number - when I do it online and hit Submit I keep getting an error. Had no problem adding my TTN to the reservation. . Anyone else have this problem? It's an Economy Standard (G class) fare. If this persists, would I be able to add it at the airport? Don't fly AC or UA much and it's a short trip but miles is miles.