This is probably the latest iteration of a long-standing problem that AA.com refuses to fix.

I routinely see it with JL flights, whether under JL numbers or AA codeshare numbers.

AA refuses to fix it because it generates revenue for them, when customers who HAVE to get the reservations in place give up and select a higher-priced option.

You might try telephoning AA and asking them to set up the reservation. They'll tell you it is subject to their phone fee, and then they will generally waive the fee when you tell them that it is currently not possible to make that reservation on the website.