In your class settings, you may notice some of assignment types appear "locked," while others appear in blue and others in gray.
Assignment types in blue are ones that you've enabled for the class, but have not yet used. Both blue and locked assignment types can be selected when adding assignments in your class.
Assignment types in gray have not yet been enabled for the class. To enable these assignment types, simply click the assignment type you'd like to enable.
Assignment types with a lock symbol are enabled for your class, and are already in use by an existing assignment. It's "locked" because you cannot disable assignment types that are currently in use in your class timeline.