How can I decrypt my HD?

I'm trying to decrypt my disk and turn off FileVault, but I can't do that. Firstly, the Turn Off FileVault... button is disabled.

Locked

Unlocked

Secondly, I try sudo fdesetup disable with Terminal but it also doesn't work. It returns FileVault was not disabled (-69595).

Edit I've also tried doing it from recovery, but it doesn't work. For more info, see the comment on @n1000 's answer.

I'm running Mac OS Mojave (Version 10.14.1) on a MacBook Air 11" (Released in Mid 2013). Thanks!

Solution 1:

sudo diskutil cs decryptVolume <disk> -passphrase "YOUR_RECOVERY_KEY"

If you don't have a recovery key created separately from your users passphrases, you can follow the instructions here:

https://support.apple.com/en-us/HT202385

Usage:  diskutil coreStorage decryptVolume
        lvUUID|MountPoint|DiskIdentifier|DeviceNode
        [-stdinpassphrase | -passphrase passphrase]
Start a background decryption process that will convert the on-disk bytes which
back the given logical volume from encrypted back to plain. You must supply a
"Disk" passphrase (not a FileVault User passphrase) interactively or with one
of the above parameters. After this command completes, the conversion will be
ongoing; you can check progress with `diskutil coreStorage list`.
Example: diskutil coreStorage decryptVolume
                  11111111-2222-3333-4444-555555555555