This process assumes you have some sales orders that can't be fulfilled. These sales orders were likely generated from within Sales.
Automatic Inventory Ordering (MRP) is the tool that generates your purchase orders.
- Sales orders must contain a "Delivery Date" in order to appear in MRP.
- The MRP tool will organize the products onto purchase orders for each product that is ordered.
Please see here for more information: User Guide "Automatic Inventory Ordering"