The issue is that there is no availability in U class ; that there are paid business seats available does not mean that there is award seat availability

Check back closer to date of departure and see whether availability opens up or change the flights to one with availbility are your options. Final chance is to try for an on-departure upgrade

Qf776 at 16:55 has business award availbility

Alternatively if keen to get business class on an evil overnight flight then if you are prepared to go via Sydney, the 23:00 departure ( QF568 ) has availability too

Dave