If you can’t turn on FileVault on Mac
If you can’t turn on FileVault for a disk, some of the formatting information on the disk may need repair. Try using Disk Utility to repair the formatting.
Open the Disk Utility app on your Mac.
In the list on the left, click the disk you want to repair.
Click First Aid, then click Run.
Disk Utility examines your disk’s formatting information for errors, then repairs any it finds. See Repair a disk.