Thank you for reaching out to us. Please be advised that Cornerstone is a page builder. You can use it to edit a posts or pages. When editing a product page using content builder, you can only edit the product description area and not the whole product page layout. The product page layout is utilizing the WooCommerce templating system which layout the structure in displaying the product image on the left, product summary on the right and the rest of the product description at the bottom.
Please go to X > Theme Options > WooCommerce > Single Product and make sure Product Tabs is enabled especially the Description Tab.
If your settings are correct and you still have problems then please get back to us with the URL/User/Pass of your WordPress dashboard using the Secure Note functionality of the post to follow up the case.
Hope this helps.