Well that sounds like the Pixhawk6 and the receiver are not recognizing each other. The electrical connection would be the first thing I would suggest checking. The wire from the receiver should be plugged in the the bottom edge of the Pixhawk6 servo rail in the first slot labeled RC IN. That slot should be for any SBUS or PPM receiver. Also make sure the wire is not plugged in upside down.
Also check the simple things like making sure your pixhawk6 is connected through mission planner via your radio link to the laptop. Ardupilot should auto detect the receiver being attached when it boots up.
https://ardupilot.org/rover/docs/common-radio-control-calibration.html
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