Hi,
It must be because there is no product image, can you try to add a product picture to that "hello" product and re-try to add to cart?
In the real store, if some products don't have a picture at all, don't leave the picture section empty but instead add the "no picture available" picture manually.
(which is this in your website:
voarshop.ems-machine.com/components/com_virtuemart/assets/images/vmgeneral/noimage.gif)
Or you can use "google.com Image Search" to find a better one, up to you.
I am 99% sure you will be fine after you add a product picture.
Regards.
Forum Support.