There is a work-around at the moment... if you've already loaded 6.0.6 final onto a radio, you can load 6.0.6 beta, and then you can load 6.0.4 which will allow you to upload Elevate. You can find the beta HERE. You can find 6.0.4 HERE.
Please note that with 6.0.6 beta, UI firmware downgrades are restricted to 5.6.15, 6.0.3 and 6.0.4
@Chris_Bay wrote: im sure elivate took a lot of business from them.
The radio is already sold so it's hard to took buisness in this case but it could slow the old technology replacement. Anyway, N vs AC difference is not enough to worth changing gears but MU-MIMO could worth it.
To download the 6.0.6-beta, you first have to sign up for the beta program. Instructions on on UBNT's forums. They're giving that out as general advice, not just ot people who normally woudl try a beta.