How to add "Add to Cart" button using Ubercart?
According to this answer:


  ... The content type has to be an Ubercart product content type, then the
  add-to-cart button will automatically show up on that content type
  along with all the other fields that are necessary for an Ubercart
  product like price, sku, etc.
  
  To make a content type an Ubercart product content type, edit the
  content type at /admin/structure/types/manage/{name of content type},
  choose the "Ubercart product settings" tab in the lower left of the
  page, select "Content type is a product", then save the content type.