MacOS Big Sur download error. This error occurs right at the end of a 12GB download. Its nasty. Explain why its happening?

How much free space do you have on your boot drive? Big Sur requires about 35 GB for successful download and installation.

Try the following:

#1 - boot into Safe Mode according to Start up your Mac in safe mode - Apple Support.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

#2 - go to the App Store, search for and download the full Big Sur installer.

#3 - run the installer from the Safe Mode.

I've had better luck goring thru the App store than thru the System/Software Update preference pane.