Some themes are not allowing the cart button as a menu in header part of the site. If you want or wish to add it as a menu list of menus.
Here the exact way to cart items into menu,
Go to Plugins > Add New
Enter the “Woocommerce Menu Cart” Plugin name and search it. You can get the plugin first just install and active it.
After that you move to Woocommerce > Menu Cart Setup .,
You can change the settings whatever favourable to you.