While creating a drop shipment PO the customer subsidiary match with the item preferred supplier/vendor subsidiary, then only the Created PO field in the item line is enabled. If the customer subsidiary is different from the supplier/vendor subsidiary of the item the created PO becomes disabled.
To enable the Drop Shipments and Special Orders feature
- Go to Setup> Company> Enable-Feature
- Click the Items and Inventory subtab
- Check the Drop Shipments & Special Orders box and Click Save