Seems this error showed up in 4.0 release also
I am running the latest beta Mission Planner.
I switched the drone on. I went to compass/motor calibration. I didn’t press the safety button. I tried to do the calibration. Nothing happened. I pressed the safety button. Nothing happened. I finished calibration. Pressed the safety button. Then did the calibration
I did it again, but this time I restarted. Pressed safety button and did the calibration and it gave the same error