This has happened to me more than once. I am Gold but have been unable to select 1A, even when it shows as available, is within the 72 hour window when it's opened up to non Golds and with half the F cabin showing as available. I received the same message as the OP. I couldn't be bothered phoning up to select it but I have been able to select it during online checkin. I put it down to (another) bug in BA's systems.