Toggle Navbar on mobile screen size not working

I have a toggle hamburger navigation bar that is turned on when the screen becomes mobile size. However, although the responsive navigation bar changes from horizontal to vertical for smaller screen sizes, the hamburger toggle nav bar does nothing when clicked. I tried various things in my CSS such as .flex-nav .open { } as well as flex-nav > ul { }. Here is the link to my Glitch code to get a better understanding.

If someone can help me to fix this toggle navigation bar to work on mobile screen size, that will be very helpful.

This issue has been resolved. Thanks.