[BScout]

The “non-upgradeable” remark makes me think this is some kind of NFR license. Or specialty education license. I wouldn’t buy a “get update” license until you contact Avid first to make sure it’ll work.
This does not sound like a standard license (how did you come to obtain it?)

As for the dmg not opening, there was a change around that time on disk images and macOS. It may not be corrupt but it may be incompatible with that macOS version and later. Even Apple had to reissue some of their older installers due to this issue. I don’t know if Avid Download Center (in your my.avid account) has repackaged versions (or if it is just the old images.) But if you can’t open a newly downloaded version your options are (A) contact Avid and see if they can send you a new disk image or (B) open that disk image on older macOS that will open it and drag the installer to a usb stick to take to the computer you want it installed.