That is what I meant. If you installed the Firmware using Mission Planer you have installed the Arducopter FlightStack.
The Command you are referring to is a PX4 (another FlighStack and what this Forum is about) Command. So it may or may not be available on Arducopter - in your case it is not.
Either check with Arducopter (Documentation or Community) or install PX4 Flightstack (using QGC and choosing PX4 when installing new Firmware).
APM = ArduCopter