I cannot decrypt file vault. I get an error message.

I got some ugly experiences with filevault. It corrupted my entire hard drive and I had to reinstall from scratch (fortunately, I had a Time Machine backup)...

However, in my case, my TM backup was also encrypted, and -- guess what -- for some strange reason, from the Recovery Utility, I was unable to mount my backup after inserting my password... Grrr..... 😠

If you have a TM and it is not encrypted, then:

1. Do a full TM backup;
2. Restart your Mac in Recovery Mode (CMD+R);
3. Format your drive with the Disk Utility;
4. Restore your TM backup.

If your TM is encrypted, I recommend that un decrypt it before trying this. Once you activate the decription, you'll have to wait for the ENTIRE decryption to complete. Look at the progress in the TM window of System Preferences.

Good luck!