A purchase order records the goods you ask a supplier to deliver and the agreed quantity and price. ME21N separates header responsibility from item data. The supplier and purchasing organization belong to the order context; material, quantity, plant and storage location identify the item you are buying.
Saving an order creates a purchasing document. It doesn't, by itself, create an inventory or accounts-payable posting. Those effects arrive through receipt and invoice steps. Check the supplier key before copying an older order, then check every item. In particular, a correct total value can hide a wrong quantity and price combination. The exercises check the individual fields and any requisition reference, rather than accepting only an order total.