What the message is telling you is that you cannot delete a Category when there may be other users on the system. You need to apply the System Lock to kick off all non admin users and then delete the Category. The System Lock is the right most option in the POV bar (it will either display Open or Locked). Double click on it, then Lock the system and you will then be able to delete the Category (you will need to be logged in as an administrator to do this)