It is not possible to book a SKU out when it is marked as Discontinued. The SKU will need to be made Active again to book the stock out.
There are some steps you need to take to be able to book Discontinued stock out:
Click Products then Overview.
Enable Show Discontinued Products.
Search for the SKU.
Click Actions then Edit.
Turn OFF the Discontinued toggle.
Click Save.
Click Warehouse then Book Inventory.
(3PL Only) Select the Client.
Select the Product.
Select the Warehouse.
Select the Location of the stock.
To see a breakdown of the stock click the UnAllocated tab.
Set the Quantity.
Enter a Comment.
Change the Action to Out.
Click Book Stock.
Click Products then Overview.
Search for the SKU.
Click Actions then Edit.
Turn ON the Discontinued toggle.
Click Save.