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

The reason may be 1) that the minimum order quantity has not been reached, 2) or it may be that at least one item in the cart does not have a selected quantity

1) In order to issue an order, Weerg requests that the minimum order amount be at least € 35 (excluding VAT and shipping costs).
This condition can occur with 3D printing only orders for which the minimum initial cost per prototype is € 9.98. Then check how the amount shown in the image below exceeds this minimum figure.

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2) Another 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 pre-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):

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