If your mobile menu on your free or premium theme is not working, it is most likely due to your site having a javascript error, this would usually be caused from a plugin you’ve installed.
To check if your website has any javascript errors (in the Chrome browser) you can right click anywhere on the site and to go Inspect.
This method of checking for javascript errors will be similar in all browsers but might be named slightly different.
This will then open the developer section of the browser where you can view the code of the website.
Change to the Console section to view if your site has any errors.
If your website has any javascript errors, you will see red text in the Console and this will be the reason that your mobile navigation button does not work, as the javascript error will break the rest of the javascript loaded below it.
The solution:
The best way to sort out this issue is to deactivate all of your plugins, and then activate them one by one, checking after each activate if the error is back.
Once you find out which plugin it is, you can either find a new plugin that can add the same functionality to your website, or contact the plugin developers to fix the error.