Late 2014 iMac won't upgrade from El Capitan to Big Sur

I have a late 2014 iMac. Lots of memory, 3 TB Fusion Drive, etc.

I re-installed from scratch, and that got me El Capitan.

Now this Mac should be upgradeable to Big Sur. But try that I may, this fails every time. I update from the App Store.

The updater downloads fine. It runs fine too, until it doesn't. "Error updating" messages, after an hour or two - every time, and I have tried countless times. Also re-formatted the drive again and started over, but with the same result.

What am I missing?

Michael