As HIDDY says, welcome to BA’s seating policy

Unless you are on a full fare ticket for the cabin you are travelling in, or are travelling in First Class, you can only get seat assignments at +24hrs from time of departure.

Personally I think the policy stinks, but on the other hand it does suit many travellers very well indeed & they’d have it no other way. But the merits or otherwise of the policy is an altogether different discussion, which has been done to death many times over here on the BA board.

Do a search on “Seating Policy” & loads of threads will turn up where you can see some rather heated debate on the policy.

However, for an explanation of the way the policy works, do have a look at the second part of the Sticky near the top of the board, entitled “The BA Upgrade Guide and BA Seating Policy Guide” where you should find all the info you need ^