Looks like you have 2 ID columns in the cart definition:
ID - this one is automatically createed for the cart object
itemID - this one would have been added.
these 2 ID columns are clashing with each other. in the add to cart button behavior on the detail page, you are not giving the itemID column a value.
i would remove the itemID column from the eCart object, then edit the add to cart button behavior to double check the column bindings.