Add to cart button not available

Hello, I wish to sell stationery or other items on my Opigno site. So, I created a new product type and referenced it to a new product display content type. The pricing, SKU, everything is visible on the product display page but not the "Add to cart" button! What am I missing here? Why is the add to cart button/form not viewable in the manage fields section of the product display content type? I can sell courses on my site but not other products. Any suggestions to get that add to cart button on the product display page?