Could be that they haven't assigned an a/c type yet. Typical TK

I would try again every few days, AFAIK a BD redemption ticket should not hinder your seat assignment.
Flying TK in Business on a BD redemption ticket this week: managed to assign seats using 'check my booking' on LOT.com a few days after the booking was made (about a month ago): however not able to do anything now - the whole cabin appears unavailable - it might well be some new stupid rule for redemption tickets?