The original poster (OP) stated both an inability to 'select a seat' as well as 'lost ...seat'. I am left with the impression that they were not prevented from obtaining a specific seat assignment when they booked, but instead have found that the inability 'to select' is essentially equated by the fact that they are finding that they are losing their seat to someone else after having previously chosen it.

This recently occurred to me on an international leg (IAH -> CDG) with my business-first (BF) seat reassigned without any notification whatsoever. I just happened to review my itinerary. As others have responded on that thread, this can occur if there is a change of aircraft (sometimes same type of aircraft, but different layout) or a flying air marshal (FAM) is subsequently assigned your seat. I imagine there are other reasons for this to occur, as well, but the aforementioned seem the most likely candidates from what I have read.