Big Sur Update

Well there is another way that could be done from where you are and no requirement to visit the the Mac Shop.

What to do before you sell, give away, or trade in your Mac and in your case - Yes not selling, trading in or give away but the net effect would be starting over as is the computer was new and never used.

Must ask before moving forwards - No Admin Account on this computer.

Hum - is this computer Owned by a Company, University, School or Organization ?? If Yes, the computer may have been Enrolled in a Mobile Digital Management program ( MDM for short ). If the answer is Yes, one would have to contact the IT Department of the Company to fix this issue. They will control everything that is installed or not allowed to be installing on this computer and we can do noting about this - only they can.

If Owed by the user - then that changes things a lot. The Internal Drive is a slow Rotational / Mechanical drive and will always be slow when compared to Solid State Digital ( SSD ) drives. So, even after we try to fix things, it is going to be slow.

To create an New Admin Account - Credit goes to this well Respected Contributor @ BDAqua from this Posting

The exact steps are as follows >>

>>> Verbatim <<< See if you can create a new administrator account by restarting the Setup Assistant:

1. Boot into Single User Mode: Start/restart your Mac. As soon as you hear the startup tone, press and hold ⌘ + S until you see a black screen with white lettering. (If you end up back on the login screen after a flash of the black screen with white lettering, enter your password and it will return to the black screen.)
2. Check and repair the drive by typing /sbin/fsck -fy then ↩ enter - as directed by the on-screen text.
3. Mount the drive as read-write by typing /sbin/mount -uw / then ↩ enter.
4. Remove the Apple Setup Done file by typing rm /var/db/.AppleSetupDone then ↩ enter.
5. Reboot by typing reboot then ↩ enter.
6. Complete the setup process, creating a new admin account.

Be very careful to notice the spaces in those Terminal Commands.

Once you've done that the computer reboots and it's like the first time you used the machine, except all your data will still be there. Your old accounts are all safe. From there you just change all other account passwords in the account preferences!!

Force shutdown by any means including Power Button.

Disconnect all external devices.

Restart in Safe Mode . It will do a Repair Disk, clear system cache files and only load Apple Software, extensions and fonts.

The loading will be slow - normal.

This may or may not allow the upgrade process to complete and hopefully successfully .

Can you explain what is meant by " Everything was lost " ??

The initial Boot Time will be slower as the new OS is indexing everything all over again and putting thing in its' new place.

Expect the machine to be slower than usual for a while as it re-organizes things.

Ugg, only works is Safe Mode. Not good. It could be Third Party Software interfering with the normal operation of the computer.

Suggestion is to boot into Safe Mode again and then >>

Suggest downloading the Application Etrecheck directly from a well Respected ASC Contributor. The application is free or paid from added features. Run the application with Full Disc Access ( Security & Privacy - Full Disc Access ). It will take a Snap Shot -  both the hardware and software. The Report will Not Reveal Any Personal Information. Post back the Full Report - copy and paste - using the Additional Text Icon ( 3rd Icon to last )

We can have a look at the report for possible issues and may have possible suggestions to resolve the issues.

Any Third Party Applications that will interfere with the normal operation of the OS, alter, modify, remove or delete or attempt to do so is an invitation for disaster and may require a Reinstallation of the OS.