You have to call. If the agent you speak to doesn't want to assign seats or says they can't, HUCA.
People have been given different "explanations" for this from agents. The last agent I spoke to acknowledged it was a glitch and said she hoped it got fixed soon; she was quite familiar with the problem.
From other reports, this may not be limited to Avios bookings. It seems to be related in some way to AA's fees for seating assignments/changes made by AA agents on third-party bookings generally, as others are experiencing the problem with paid bookings made via an OTA. Hopefully the dust settles soon and the functionality returns