Django ecommerce: Reverse for 'cart_add' with arguments '('',)' not found. 1 pattern(s) tried: ['cart/add/(?P<product_id>[0-9]+)/$']

Not that I can see or think of. This form tag is rather standard, there’s nothing in it requiring anything special. Nor is it saying that there’s a problem with rendering the page.
It’s telling you that you’re trying to render that url tag either without a product object or a product object without an attribute, key, or method named id.