It could mean many things. Sudden surge in bookings and no seats left for pre-allocation (but don’t worry, there is 99% chance everyone will fly on the day), Ba have notionally allocated seats already, there is a change in aircraft configuration so seat map is temporarily blocked or even system is partially down. Try again later and see if it works, if not maybe call them and ask.

out of curiosity (not criticism just curiosity) if you wanted to pay for a seat why did you not buy it when it was showing as plenty available? Were you hesitant and just worried now because you fear the seat map means it will be full? We’re you Hoping for seat cost decrease? Something else?