There is no lock on this screen. There is only a lock if the setting requires an Administrator (someone with the role of Administrator) to make the changes (i.e. Security). If anyone can make the changes then there's no need for a lock.

Make sure you're logged-in with the role of Administrator to see the preferences that you can lock/unlock.

There is no lock on this screen. There is only a lock if the setting requires an Administrator (someone with the role of Administrator) to make the changes (i.e. Security). If anyone can make the changes then there's no need for a lock.

Make sure you're logged-in with the role of Administrator to see the preferences that you can lock/unlock.

EDIT: based on OP's new comments:

• Log in as a user with Administrator rights.
  
• Open System Preferences and select Security & Privacy.
  
• Click the lock icon and authenticate.
  
• Click the Advanced button at the bottom and enable the Require an Administrator... in the popup window.
  
• OK the change and then lock the S&P preference pane.
  

Now when you open the Sharing preference pane you will have your lock back.