This is a very, very common issue and there are various fixes. One solution is to book the flights firm after which go back to into Manage My Booking and it will then let you reserve the seats for free.
Also, if I'm not mistaken another person reported that you can add the seats on for the fee at the time of booking but when you go to pay it will not charge for the seats, but I haven't tried this method myself. Either way, Golds and other relevant levels of status still have free seat bookings.