macOS: Finder constantly restores "Desktop" favorite under Sidebar
Have you tried using First Aid? If there are problems, you may want to use the Disk Utility Tool from Recovery mode. If you reboot and hold down command⌘+r you will boot into your local repair disk, or if you hold down command⌘+option+r you will boot into the internet recovery mode (you will, of course, need internet access and will want to give it a while to load all the options up.)

Also, try zapping the NVRAM by holding down command⌘ + option + p + r when rebooting?    

You may also simply need to trash your sidebar preference files:
/Users/Library/Preferences/com.apple.sidebarlists.plist
...or even the Finder preference file:
/Users/Library/Preferences/com.apple.finder.plist  

Also check /Library/Preferences/ and /System/Library/Preferences/ to see if there are duplicates.

Note: If you are not the owner of the computer, speak to the owner/admin before trying any of this and request permission from your system administrator. If you are the owner you may need to assign this login account admin status.