RE: Location Code must be equal to 'DROP' in Reservation Entry
If this is a Drop Ship, then the Location Code on both documents must match. Since the Sales Order is going to consume the inventory as soon as it is "received", it has to pull from the same Location. This means you cannot change the Location on the Purchase Order because it is populated by the Location established on the Sales Order.
If you really wanted it changed, you would need to delete the PO, change the Location of your Sales Order, then recreate the Drop-Ship Purchase Order.
You can reach out to your partner for any further information on the Drop-Ship process.