Unable to unlock System Preferences in High Sierra
I found a working solution to this on this Apple support community thread, Can't install programs and unlock settings:


  OSX apparently needs to regenerate a fresh auth.db from /System/Library/Security/authorization.plist.
  
  This can be triggered in terminal with the following command: sudo mv /var/db/auth.db /var/db/auth.db.old
  
  After rebooting the system everything works as it should be.