If you use Pio with "Partly shipments" disabled, the orders with missing stock items will still be synced to Pio, but not added to picking queue.
Out of stock orders in Admin Dashboard
The Issues section on the Admin Dashboard has an overview of the number of orders with insufficient stock. Click the link to find the pre-filtered list of orders with missing stock.
Filter orders with missing stock
Navigate to Admin > Orders and choose 'Filter':
Select Stock - Insufficient stock:
The filtered list will show all orders where one or more items are sold out.
Manually add available products to picking queue
If you want to manually add available products in a specific order into a picking queue, click 'Queue items in stock' button:
See more details on how to toggle 'Partial shipments' on/off here: Partially fulfilled orders