There are frequently misleading error messages in this area and it can be quite tricky to work out if there is a problem or not. A lot of forum members have reported this or similar problems, and some COVID related messaging also creeps in for USA residents, so really out of sync now. I regularly get this message too, however in my case it actually means "Your A2 visa number needs to be entered into FLY because we can't carry it over from Amadeus". And sometimes this message shows when in fact it means "just because you live in Northumberland, don't expect us to open up OLCI early just for you".

So the best tactic is to try the App OLCI, ensure the new contact address for night 1 is showing, you normally get past the baggage selection screen and then it stops with an error message. You best leave it a couple of hours. then if that doesn't work just leave it until check-in and it should all work out. If starting in LHR T3 or T5, just use one of the self service kiosks, selecting Passport Number and using the 3 letter code for the initial LHR to USA sector (not your end airport necessarily). So LHR-IAD-MEX would be IAD not MEX.