“Add to cart” is an element created for the WooCommerce product single page. This means that you need to use this element only in templates made with the theme builder that are applied for the WooCommerce product single page
You can create a new template from the themebuilder
Apply the template to all products
Add a body component
Then edit it and you are ready to create your single page with the “Add to cart” element
Useful Tutorial on how to create a WooCommerce Product page
