I expect that VA is only an Amex partner for Australia card holders. Unfortunately Australia was locked down hard during the pandemic and demand is still very high (particularly in premium cabins) for the seats available so award flights (to/from Australia) are as scarce as rocking horse poop.

Yes - try cities other than Sydney and maybe check SQ - I've had a bit of luck with them. But this may be a case of a cash ticket being the most likely outcome.