You’re using the Off Canvas panel for mobile – and there is no Menu assigned to it – you need to go to Appearance > Menus, edit your menu and include Off Canvas in its Locations.
Not sure if I’m getting it, but I have the menu assigned to Main Menu and Slideout Navigation, but the subheaders dropdowns do not work yet.
The menu plus add-on is activated, nevertheless, this option is not appearing on the menu settings – in the display location I only have Secondary menu, Slideout navigation and Main menu.