Cannot turn off FileVault process in terminal or DU in macOS High Sierra

Solution 1:

If you're on a laptop, make sure it's plugged in as FileVault will pause on battery. If that's not causing it, you can create a clone of your system using Carbon Copy Cloner (http://www.bombich.com/) or SuperDuper - that's what I would do - and then wipe the system, re-install the OS (High Sierra isn't that bad). Then I would enable FileVault on the clean system, wait for it to finish encrypting what little is there (OS only, a few gigs), then restore your files from the clone manually, then enable Time Machine and have it do its backups.

Solution 2:

Error: This command requires root access.

You'll have to run as root. Try:

Sudo fdesetup disable

I'd try plugging into a power adapter and shutting down/powering on before making an image of your system.