Right, you have an order but you just found out that not all the products are in stock. The most important thing is to inform your customer about the problem, cause he is probably waiting.
We advice you not to start a delivery process already, and wait for a confirmation from your customer to receive a different product.
In the article here you can find more about cancelling a delivery.
Important notes:
Make sure that you always notify your customer about products not being in stock.
Never start the delivery process already, because sometimes the delivery rider is there within 5 minutes, picking up the products that aren't there.