As part of the Purchase Order process, once the order is fully created, you need to mark the order as placed. This sets the order as being placed with the supplier and means that it will now appear on the Goods In screen, ready for you to receive the stock into the system when the delivery arrives.
First, open the Purchase Order in question:

Click Mark as Placed towards the bottom of the screen:

If this is not done, you will not be able to receive the stock on the Goods In screen when the order arrives.