Did you mean that you've got to run a scan from a DRIVE other than your boot drive, i.e, an external (which I"ve already disproved).
My understanding is that it can be done from the Recovery Drive or from another bootable partition such as TTP's EDrive.
How about another macOS bootable partition?
Under any circumstances, though, as I've said, I've run the command on my boot drive from my boot volume in macOS 10.14.5, so... Got me beat!
