The "Item not found" message is normally returned along with a 404 HTTP response. It means that you requested an item but it was not found on the server. There are various cases where this might happen, for example:
- when requesting an item by ID and no item with this ID exists in the database
- when requesting a device registration by the HardwareID of the device and no such device is found in the database
There are other cases as well. I am not sure what it meant in your case, as I cannot understand exactly what you were trying to do from the description you gave.