Originally Posted by
continentalclub
I assume that you're at the pre-confirmation stage, and haven't actually booked. If so, there's a known bug that tries to charge for seats, even when status has been applied.
Ignore the option at this stage, complete the booking and then go back into Manage My Booking. You'll be able to select your seats for free then, according to other posters' experiences.
That solved the problem. Thank you.