As part of the Purchase Order creation process, once the order is fully created, you must update its status to 'Mark as Placed', this sets the order as being placed with the supplier and is now ready to be received and will now appear in the Goods In screen.
From the main Purchase Order edit screen:
Click 'Mark as Placed' in the bottom right hand corner:
If this is not done, you will not be able to retrieve the order from the Goods In screen and receive once the order arrives.
