I was having the same problem, but after a few bookings realized that the only time it was happening was when I was making a reservation for my employees, not traveling myself as one of the passengers. It offered me seats in the front rows of Y, normally not open for pre-selection by non elite level members. When I actually chose seats in those rows and clicked "continue" it was as if the booking engine finally figured out that they couldn't pre-select those seats when not traveling on the same ticket as an elite level member and I'd get the error message. After realizing that, I started choosing seats further back for them and the problem was solved.