Could it be that its because VX and Alaska does not have a "live booking" and the last step checks for award availability and somehow the numbers don't appear to match in terms of A Class F Class 0 but Alaska shows as 1 seat being available and perhaps someone on Virgin booked the last seat just as you were about to complete the booking?
Just a thought. Perhaps others can weigh in especially since VX/Alaska are still integrating their systems.