If you do not see the 12.6 installer on your Avid Account, you need to go to iLok License Manager to check what is the RELEASE LIMIT date on your license. You cannot auth newer versions than that.
For example my license bundle says "Versions released on or before 23.03.2021" on the Release Limit field.
If you think your license is good for 12.6 then just to to
Avid Download Center and download it from there.