There's a problem with China flights. I had a similar problem with revenue bookings to Shanghai a few days ago on the KA front-end. Always giving me availability and then giving an error after I'd gone through all the hassle of entering the details.

The e-service centre managed to sort it all out for me, but it took about 45 minutes on the phone while they tried to get the systems to sell me some tickets! I recommend giving them a call - they are always excellent in my experience.