Please check how it works with the default WordPress theme just to check whether the issue is with our theme or not. Also, please check the site with our theme but only with the theme default plugins activate, deactivate all the 3rd party plugins from the site.
If it works fine with the default WordPress theme then provide the admin panel login details of the website to check it myself and help you out accordingly.
Your issue has been solved. I have deactivated all the 3rd party plugins from the site and activate only theme-required plugins on the site, now the add to cart button is working fine, and adding the product in the cart, please see the screenshot: https://ibb.co/wd04g9q
Now, please activate the 3rd party plugins one by one and check which plugin is creating the issue for you.