I'm not sure what you are trying to accomplish. I reviewed the scheduled tasks on my RPi2 by just entering schtasks and I don't see an entry for Microsoft\Windows\IoT\Startup. Hence, if I run the same command as yourself, I too see a file not found error. This is what I would expect to see.
What is it you are trying to do?
Mark Radbourne | Microsoft | Windows 10 IoT Core Insider Preview Support | This posting is provided 'as is' with no warranties and confers no rights.