Only products that have been enabled as Purchasable can be added to open (Parked or Placed) Purchase Orders (PO's).
If a product is not showing up in the PO's "Product" search field, it may be because the product has not been setup as Purchasable. To update the product to be Purchasable:
Go to the product's record
β
βIn the Details tab, under Type, check that the Purchasable toggle is on (green).
βClick Save.
You can also update multiple products to Purchasable using the Product Import function. For more details, see Import/Export Products.