You cannot close a PO once it is created. Only if the PO has been Approved, and payment has been done, then it can be closed. Check the purchase process flow below,
The Purchase module streamlines the procurement of required components and allows you to add them as unassigned assets. The Purchase module is tightly integrated with the Inventory. You can configure the vendor-related information and the products supplied by those vendors and their price quotes in ServiceDesk Plus – MSP . This information helps in making the choice of vendors while generating the Purchase Order (PO) by comparing the pricing of each vendor. The purchase process starts with a purchase request and is completed when the requested component or item has been purchased and reaches the requester of the purchase request. Below is the workflow that details the various steps involved in a purchase process.
If you want to edit the created PO , then in Actions click on Edit , do the changes and save. Else you can delete the created PO.