On delivery order page, at "Additional Info.", you need to go to "Shipping Policy" and select "When all products are ready". Then the "check availability" button would show up if any item in the delivery order was not available. 

Hi Julian,

I know that if you are trying to transfer an item that is incorrectly set as a consumable and not as 'stockable item', check availability will not appear. No stock was booked into the item as it is a consumable, and it would not check availability.

Hope this helps!

Hi,

The ‘check availability’ button will appear in ready state. But there is a condition that if any one of your ordered products are not in stock.

Regards