It looks like the article that you are trying to edit was "checked out". Sometimes this can happen if an administrator opens an article and then leaves the page (closes their browser tab or hits the back button) without "closing" or using "save and close". This leaves the article checked out to that user. As an administrator you can "check-in" the article by clicking the small padlock next to the article title.
Careful! If you hover over the lock button it will tell you who last checked the article out and when. If you check-in an article that another user is still actively editing, none of their changes will be saved.