That's probably because you set "Display 'Add to cart' button" to "No" when configuring that module.
In the backend, go to -> Display -> Content modules, and find the module with ID #140. Click on its name, then look in the right pane for the aforementioned parameter and make sure it's set to "Yes". Save and check your site, should be good.
Presently, this seems to be your only product listing in the site. For new ones, see that you have the button enabled if you want that to be the case - you can determine that as default in the system configuration, so you won't overlook it.