Welcome to FlyerTalk
I suspect your wife’s booking is sitting in a queue waiting to be re-ticketed. If so, she will not be able to pay for an advanced seat assignment until after the ticket has been re-issued.
Contacting BA is the best thing to do. However, if your tickets were booked through a travel agency and the scenario above is true, you will need to get in touch with the TA first and have them re-issue the ticket.