Cannot update due to "shim-signed"

[stevef]

It appears that your system is missing a  script (named grub-multi-install) which is called by the shim-signed upgrade process.  As the script is missing, the upgrade process crashes.  The script location is part of the protected system area that isn't usually touched so while it may be possible just to copy the script back to the system, we really need to understand why the script isn't where it should be.

Other things may be wrong as well and the AI advised attempts may have had adverse effects.

Are you aware of anything just before the problem started that may have affected your system files ?
Do you have a System Restore Utility (aka Timeshift) snapshot taken before the problem started ?
Would you be prepared to back up your data and re-install the system ?