Does Contact Editor Extension work with Version 5.6.1?
This happens when the menu link is not registered properly for all extensions. I always have a habit of performing cleanup cache and some times delete template_c and rebuild menu.

In your case as @Tony said after performing cleanup cache the menu should be registered and the extension should work. You can do this by navigating to CiviCRM >> Administer >> System Settings >> Cleanup Caches and Update Paths >> Cleanup caches.