I think I have a similar query.

I have a flight on BA from LHR to ABZ. Booking class D. Ticketed by QF after a flight on QF9. It's now within 7 days of my flight. QF MMB won't let me select seats on the BA flight. When I log in to BA MMB, I can go through the motions of selecting seats, but when I go to save the changes, I get an error message. It says "Sorry, we are unable to process your request. Please contact your local British Airways office." I'm not super worried. I'm travelling with an elderly relative, but it's only a 1 hr flight. So if we don't sit together, it's not a big deal. But I'm also curious to know why I can't select seats.