We're unable to print a packing slip from the mobile app at the moment. For now, you can log in to the web version of QuickBooks Online and print it from there. Here's how:
Go to the Sales menu and select Customers.
Select the customer's name to display a list of their transactions.
From the Transaction List tab, select the box next to each invoice that you want to print a Packing Slip.
Click the drop-down list for Batch actions, then click Print packing slip.
Let me share these links and articles for additional guidance when using the app: