Brightongold
The availability of club seats does in no way mean you can upgrade using avios via MMB.
An upgrade using avios is only available when BA release redemption seats. It is a black art to try and second guess when this may be.
It is more likely, though not guaranteed, that redemption seats will be released the nearer the actual time of departure -14 days perhaps. BA will only do this if they feel they cannot sell the seats, and MCO is a popular destination as another poster has hinted.
My recommendation is to keep on trying, evermore frequently, as your departure date nears.
For example, my last 3 flights to BKK I have managed to upgrade, from WT+ to Club, at least one of the legs within 10 days of departure.