Trying to upgrade from Catalina to Big Sur on Mac Desktop

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 3-5 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 System/Software Update preference pane and begin the download.

#3 - run the installer from the Safe Mode.

I've had better luck from the Safe Mode because I evidently have some 3rd party software that interferes with the installation.

Check in Applications folder for " Install Big Sur " application. If one exists - Drag and Drop to Trash and Empty the Trash.

Then use the instructions of @ Old Toad to download preferred method via Ethernet Cable to Router.

Try a different way: assuming you have already checked your entire sw and hw is compatible with Big Sur, boot in Recovery mode (hold CMD+R at startup) and install BS onto Catalina.

HTH.

.