The problem is that you are trying to access the /cart/ URI on an AJAX order form template. The AJAX order form templates don't have a cart template because the AJAX template represents a single page that makes AJAX requests for most of the checkout process (including the cart), so there is no cart page.
I'm curious how you got to that page, though, are you linking directly to it? If so, take a look at how to link directly to a product.
If you want that link to work, you can update your order form to use one of the Wizard templates instead.