In the Sales Order window, on the Logistics tab, you have selected the Purchase Orders checkbox to create purchase orders for the items that appear in the sales order automatically.

In the Sales Order window, you have chosen the Add button and the Purchase Order Confirmation window has appeared.

In the Purchase Order Confirmation window, you have chosen the button to shift all the rows from the Sales Order table on the left to the Purchase Order table on the right, but you have not specified a vendor. Therefore, a purchase order cannot be created.