Turning on FileVault fails on MacBook Pro with 'disk in use by driver' error
You can try using the command line version, fdesetup. The simplest method of doing so is opening Terminal (in /Applications/Utilities) and typing 


  sudo fdesetup enable


Once you put in your username(type it exactly as shown in the terminal window before the ":~" ) and password, it will give you a recovery key. Write it down and double check it! This is your only way back into the system if you forget your password. 

Reboot the computer and the process should begin. 

If that gives you the same error, you can try doing a deferred process. 

NOTE TO ALL:

Do not do this method if you have a mac with a Fusion Drive in it. 

To do so, open Terminal and type: 


  sudo fdesetup -defer ~/Desktop/recovery.plist


This will run full disk encryption at reboot, so everything will be closed. The user you are logged in as will be the default username/password used to unlock your encrypted drive.  Since this method will not display the recovery key, it will be saved on your Desktop as "recovery.plist". Treat this as your password, it is just as powerful. Once you reboot, write this down somewhere safe. 

Hopefully this works out for you, and as always, be sure to make an unencrypted backup before you do this, just to prevent any data loss.