The complete pre-order button remain gray, how can I continue?

The reason may be that at least one item in the cart does not have a selected quantity

A reason can be given by the lack of selection of a date or quantity as in the example below: therefore, having no price to assign to the item, it is not possible for the system to continue with the order until you choose a price/date.
You will be presented with a situation similar to the one below where the item in the cart has neither date nor price (red boxes):

Why does the complete pre-order button remain gray and I cannot continue? 02