Generally the requirement of pre closing the purchase order comes from the business frequently due to several reasons like the vendor has stopped supplying suddenly or that material is not required any more in the future etc. I have provided the document here to close Purchase order. Once you close the Purchase order, it won’t be allowed to do Goods Receipt.
Closing Purchase Order -No future GR:
Once we activate the flag for Delivery Completed field, system won’t allow to do Goods Receipt for this Purchase Order. Delivery Completed indicator is available in the Delivery tab of the Purchase Order item.