That's correct. In the past, when booking a United Award, you can see the THAI PNR on United site. I would then go the thaiairways.com and retrieve my award booking using the THAI PNR.

Years ago, it would go through with no error. Maybe in the last year I would sometimes see an error, but it would go through and the seat change would be indicated on the United site under my primary award reservation.

But this time I get the error and the seat change does not go through. I posted my experience because in previous posts people seem to be able to complete the seat selection, albeit with a few days delay. I'm not seeing anything.

However at this point, the seats I wanted are no longer available. My current seats are not too bad, so I will just hope that next I will be able to select seats again.