Your theme code seem to be missing lines 1 and 2 in this file:
Restore them and it will fix your menu.
Do not try to fix it with CSS or JS code – there are other areas which depend on the js class set on html element.
Duplicate of:
- Header, Main Menu option in Mobile view is not working - #11 by tim_1
- Hello my navigation has completely disappeared from my mobile website - #2 by tim_1
- Hamburger Menu on mobile isn't working
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