Please go to your wp-admin panel and follow the instruction for add mobile menu:
- Go to Appearance Menu > widgets
- Here you found some widget position, please go to offcanvas widget position
- Add a custom menu widget here
- Please select which menu you want to show as mobile menu
- Then save it
- You are done ? please check your site from mobile/make your browser small width
You can add many widget in this position as you like, for example: search widget, social share etc.