How do I turn off FileVault when the button is disabled?

I'm trying to turn off FileVault so that I can reinstall macOS. However the "Turn Off FileVault..." button is disabled and has remained so since first trying several weeks ago. How might I enable or bypass the button in turning off FileVault or how might I reinstall macOS without turning off FileVault?

2 Answers

The MacBook Pro belonging to my sister in law was running High Sierra. She was having a lot of problems and was advised by someone from Google to reinstall the system. The reinstall halted with an error message something to the effect that it couldn't install because the disk was being encrypted (She had at sometime in the past turned on FileVault). Now every time she starts up or restarts she gets the same error message. Perhaps the issue could be solved if she could turn off FileVault but the 'Turn Off FileVault' button in the Security & Privacy panel of System Preferences is disabled and has remained so.

According to this Apple support page, you turn off FileVault by

1. Choose Apple menu > System Preferences, then click Security & Privacy.
2. Click the FileVault tab.
3. Click the locked icon (in the lower left corner of the window, denoted "Click the lock to make changes"), then enter an administrator name and password.
4. Click Turn Off FileVault.

The "Turn Off FileVault..." button is disabled because you haven't clicked the locked icon to enter your password.