I had the same error when trying to book a multi-city itinerary (A-HKG-B) with both sectors in J and a long overnight transit.

Both sectors were available when searched individually, transit in HKG <24 hrs, but the full itinerary doesn’t show up when searching one-way from A to B. It only showed options with shorter transit time.

Called in and CSO was able to price out my itinerary, but charged a 40USD extra handling fee. I asked him to waive the handling fee because itinerary couldn’t be completed online due to this Class Y error, he initially said no, though he escalated to supervisor and they managed to waive it. My booking is now completed. (yay)

OP you might want to call in, but be prepared for extremely long wait times. Good luck!