I suspect that you have one or more Cathay segments. CX uses a proprietary booking engine. Even though the tool shows availability on CX on the flight you want, when the tool tries to actually confirm the choice with CX when you ask it to book it, you get the error message.

Same thing happened to me. I contacted the oneworld booking desk and they came up with an alternative (fly BA--yeah right). I booked it with the BA segment and then called CX, which said there was availability, and they changed it back to what I originally wanted.