I can't delete Guest User

See here and have zero need to even attempt to remove.

Should the Guest Account be removed ( which it can not ) and the computer is stolen and turned on with the Guest Account ( no password required) the computer can be tracked via the Find My application built into the Operating System and Enabled by the user ( you )

If, in reality, you just want it to not appear during the sign-on process, instead of deleting the ability, just uncheck the options for it to appear as an option during sign-on.

I can't either; seems to be intentional by Apple. But then I see no need to delete it.