How to enable journaling to be able to upgrade to El Capitan?
Warning: before trying my solution, ensure you have a complete backup of your hard drive (usually Macintosh HD)

Before trying the method below, try the following:


Open Disk Utility (booting to Recovery Mode by holding cmdR while starting your Mac, select Disk Utility in the OS X Utilities menu)
Select your main volume (usually Macintosh HD)
Select First Aid -> Repair volume... then wait for the operation to complete.
Go to File > Enable Journaling


If the above does not work, try

Erase your drive and install OS X

(From Apple's website)


  You can use these steps to erase the files on your startup disk and install a new copy of OS X:
  
  
  Before you begin, make sure your Mac is connected to the Internet.
  Restart your Mac. Immediately hold down the Command (⌘) and R keys after you hear the startup sound to start up in OS X Recovery.
  When the Recovery window appears, select Disk Utility then click Continue.
  Select the indented volume name of your startup disk from the left side of the Disk Utility window, then click the Erase tab.
  If you want to securely erase the drive, click Security Options. Select an erase method, then click OK.
  From the Format pop-up menu, select Mac OS Extended (Journaled). Type a name for your disk, then click Erase.
  After the drive is erased, close the Disk Utility window.
  If you’re not connected to the Internet, choose a network from the Wi-Fi menu.
  Select the option to Reinstall OS X.
  Click Continue and follow the onscreen instructions to reinstall OS X.