Does the ffu file exist at the specified location, and is it for your device?
My phone is a Lumia 635 Gophone ATT RM-975_1005. As per the debranding tutorial, changed the operator and the manufacturer model name. After that; I flashed an unbranded rom country variant of a RM-975_1030 that was the only one I was able to find.
The rom is already flashed to the phone and working properly. I do not know the location of the rom in the phone more than the route you gave to input in the comand line.
Thanks in advance.