install Big Sur

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.

What year and model is your Mac?

How much free space do you have on your hard drive?

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