I believe the sort button isn't showing because you're using measure values. Tableau can't decide how it should sort, so it hides the option. You can still sort by right-clicking "Driver", selecting sort and specifying by which measure to sort.
Hope this helps!
Also can't right click on driver when looking at viz on website.
I swear it worked previously. Could the new version of Tableau have a bug? 10.1.3 Public version?