Setup Assistant Runs on Every Restart
Does this happen on another user's account on this Mac? If you don't have other user accounts, go ahead and create one to test the problem - you can always delete it later - to see if the problem occurs on this new account. If it doesn't occur, then this sounds like the permissions on your user account are all munged up and you need to try the first method below. If the problem occurs with other and/or new accounts, or recurs after trying the first step with your user account, then try the second method.

The first method is to attempt to fix the permissions on your own user folder:


From the Finder menu bar, choose Go > Home. Your home folder opens.
Choose File > Get Info. An Info window for your home folder opens.
If the Sharing &amp; Permissions section at the bottom of the window isn't open, click the triangle  in that section to open it.
If the lock icon at the bottom of the window is 'locked', click the lock and enter an administrator name and password.
Click the action menu (Gear with downward arrow) then choose “Apply to enclosed items.” Click OK to confirm the action. 




The 'Apply to all items' process may take quite a while, especially if your user account has many thousands (or hundreds of thousands) of files. Once it is finished, restart the Mac or logout from your account then login again.



If the first method does not fix your user folder permissions, then you need get more drastic, you need to reinstall macOS from macOS Recovery. Details about how to do that at the Apple Support page.

Lastly, if the macOS Recovery method does not work, you need to have a backup of your Mac, then erase the startup disk, reinstall macOS or restore from backup.


Methods derived from this Apple Support page.