Disabled Turn Off FileVault... Button

Hello, you need to first unlock the lock in the lower left hand corner. The user that unlocks that lock needs to be the FileVault user that can unlock the Mac when you boot it.

Specify that the drive should be in an encypted or decrypted state takes only a moment. Once you make a change, your Mac will slowly and deliberately, over the next DAY of awake time, read each and every used block on your drive, and write it back in the selected state (encrypred or decrypted).

If you have recently made a change, you now need to WAIT until the process has completed re-writing your drive. Only time AWAKE can be used for this, and your Mac can continue to use the drive in a half-decrypted/half encrypted state for normal work.

--------

Normal operation of MacOS with an encrypted drive is that encryption/decryption is performed on-the-fly, with no measurable additional time needed for encryption/decryption. There is no need to completely decrypt your drive to perform a system Upgrade, unless you wanted a different encrypted/decrypted state when you were done.