Hi Alice,
Welcome to the Webflow Community!
The issue lies with the right hand side margin on your navbar on the desktop and tablet breakpoint, which is a huge negative number. The menu is showing when you click the button, but it’s off screen. Remove this margin, as the navbar should be the width of the viewport.
Reply here if you have any problems and I’ll be happy to help. 
For future reference, try and avoid negative margins at all costs. They are unnecessary, difficult to debug later on, and will cause responsiveness issues further down the line.