The disk "Macintosh HD" can't be unlocked notification

First thing to do is immediately make sure you have a good current system backup of your data.

After you have a good working backup boot into Recovery Mode (Command + R) or Internet Recovery Mode (Command + Option + R) and launch Disk Utility. Within Disk Utility click on "View" and select "Show All Devices" so that the physical drive appears in the left pane of Disk Utility. Run First Aid on the physical drive first, then the container, then the actual Macintosh HD and Macintosh HD - Data volumes to see if they can be repaired. You may need to unlock/mount the Macintosh HD volume before you can run First Aid on it.

If Disk Utility cannot repair the drive, then you will need to erase the whole physical drive and reinstall macOS or restore from a backup.

Then I would suggest turning Filevault off. Once it is finished decrypting the drive you can turn Filevault back on. Only do this after you have made sure you have a current backup of the system.

How long did you wait after turning it off... it can take days or weeks to finish.