Error 21916799: SKU does not exist in Non-ManageBySKU item specified by ItemID.

This error occurs when you revise the inventory status of an SKU with a zero quantity. Typically, this occurs after the SKU was changed to a zero quantity using a ReviseInventoryStatus request (effectively removing the SKU from the listing), and you attempt to update this non-existent SKU. To fix: Re-add the SKU using a revise item call with the necessary details (such as SKU, price, and quantity). After the SKU is successfully added to the listing, proceed to update its inventory status.