When we get a sale order for a product, Odoo creates the transfers for Pick and Delivery with no issues. We have our Product Reservations set to "manually" because it works better for our business model. When we go to reserve a product and click on "check availability" on the Delivery Order step it does nothing, there are plenty of product in stock and they are in a location that is able to be picked. I manually added stock to the Output location because after reviewing the product page I noticed there was no stock in the Output location, almost like stock either vanished or did not transfer and retried check availability, it still did not work. I made sure that the Destination location for the order picking was set to the Output location. I am now at a loss, any help would be amazing.
First you have to complete the pick operation before going to delivery operation. Please confirm whether this is done. And also check the qty on hand in inventory report just to confirm the qty in required location.
In that case, one possibility could be that the product which is there in output location might have got reserved/utilized for some other order. In inventory report, compare the qty on hand and available qty for that product in output location. If available qty is not equal to qty on hand then the product has been reserved for some other order. Also, product moves can be analysed to trace the movement of the product.