Hello,
We checked adding to cart on your site with the default theme and found that it also does not work https://gyazo.com/7e22f58b39d7bdf6150d641571130a01 So, our theme is not the reason of the issue. Contact WooCommerce support to get help with this issue https://wordpress.org/support/plugin/woocommerce/ because we are able to help only with issues caused by our theme.
Regards
