"No administrator was found" in Startup Security Utility
I had a similar issue with my 2018 Mini: I had to delete the .AppleSetupDone file and run the "Welcome to Mac" routine, creating a new admin user there before I could alter SecureBoot, even though the disk had been imaged with an admin user. 

Peter Thorn's answer here gets to the exact cause of the problem and provides the solution for a working Mac that has MacOS installed.

If you don't have MacOs installed, you could try booting it in Target Mode from another Mac, which I understand bypasses the Secure Boot! (Though if the drive is encrypted, you'll need the password, unless you're wiping it anyway.)

Alternatively, take it to an Apple Store. The SecureBoot settings can't be changed unless there is a valid admin user, so you can't boot to an external. Recovery is another option, but you say that's not working.

PS. I'd love to work somewhere where you get to keep your laptop when you resign!