"message": "We didn't find the resource/entity you are requesting. Please verify the request"
There might be several reasons for API failure. One of the reasons is that the item is listed either from eBay UI or by Trading API, so you are not getting any result in the getInventoryItem call. You will get the inventory records of a seller's account by bulkGetInventoryItem, getInventoryItems and getInventoryItem call, only if you have listed the item or created the inventory record from inventory API. Listings created outside Inventory API will not be visible via Inventory APIs.