In past I had similar experience with Swiss, it was showing available on UA and when I try to buy, comes "rapidly changing message". I had called UA thru call back link. rep saw the same thing and she did book me manually the same flight. though it was not ticketed for few days, as it was all done manually, but eventually, it did show up and I got ticket as award ticket, in spite of it "really" not available and UA site showing it as error. Key is to ask agent "As it shows available and we both cannot book it, can you make a manual reservation and send reservation desk to manually issue the ticket as award ticket?" that worked for me. YMMV.