To get mobile offcanvas menu, you have to publish menu module on "offcanvas" position.

1. Duplicate the Mainmenu module and publish the duplicated menu on module position off-canvas
2. Now you have the same Main Menu module on module position off-canvas.
3. But of course You can pubish any menu on module position offcanvas. This module position will be shown in responsive view. "Offcanvas" position can be used also for other modules (flags, search, tags etc.).