Thanks Lander, sorted out but some clarification maybe interesting for you.
I failed to mention we are using PXFMini and Pi Zero W and above should have been sudo apt-get install apm-copter-pxfmini opposed to erlebrain.
Problem; using QGroudControl (Windows10) ROVER comes up armed and message "This operation cannot be performed while vehicle is armed' when attempt to calibrate sensors. When disarm is attempted message pops up 'Arm/Disarm command failed', that's where I was stuck.
Solution; change parameter ARMING_REQUIRE from default 'Disabled' to 'THR_MIN PWM when disarmed' allows calibrate sensors, radio, etc., OK.
Changing above parameter back to Disabled still gives Arm/Disarm command failed, no clue why...