Radio calibration failure

I’m STILL trying to calibrate my transmitter.
Nobody responded to my previous post.
My setup is:
Spektrum DX9 with all servo/channel assignments defaulted.
Orange R820X V2 connected to Pixhawk 2.4.8 via PPM encoder
QGroundControl flashed firmware to Pixhawk (at least twice).
The components - Pixhawk, receiver, battery, power module, Mini OSD, FPV module, GPS, buzzer, button, PPM encoder, ESC and motor, and one servo are all mounted on a sheet of cardboard to simulate installation in a vehicle.
The components are all connected in accordance with the labels on the Pixhawk.
When connected to my computer with a USB cable, the Pixhawk blinks all its LED’s and plays its merry tune as it does when powered by the battery without the USB cable.
When the battery is connected, the ESC/Motor beeps the appropriate sequence to indicate the cell count and that it’s connected.
QGroundControl’s Vehicle Setup screen displays a message that “Vehicle did not respond to command MAV_CMD_PREFLIGHT_CALIBRATION”.
"OK"ing the message brings up message to center trims, but "OK"ing again brings up a “Detected 0 radio channels, To operate PX4, you need at least 5 channels”
The transmitter is bound to the receiver, as evidenced by the ESC/motor working normally when connected directly to the receiver.
Is there anybody that can give me a clue what to do next?

As you have noticed yourself the issue is with the receiver/transmitter. These two are not connected as they should be, and it is not really a PX4 issue. My best guess is to try and find the datasheet for the Orange R820X V2 to try and see how you link a transmitter to it. Normally there is a green LED on the receiver which will indicate the connection status.
I have personally had the same problem, where it turned out the receiver and transmitter had different software versions, since in the US and EU they are to transmit with different frequencies. So if the receiver and transmitter are not setup to the same, they can’t connect.

Thanks for the advice and suggestions.
The transmitter and receiver seem to be talking to each other. Their native protocol is PWM, so the PPM encoder is presumably necessary to convert the signal to one compatible with the Pixhawk. The LED in the receiver indicates connection and, when I disconnect the PPM encoder from the receiver and connect the ESC/motor and a servo directly to the receiver, they both work properly, responding to stick motions on the transmitter as they should.
Is there a way to verify whether the PPM encoder is sending appropriately configured signals to the Pixhawk?

That is unfortunately outside of my expertise area. You will have to find someone else to answer that. I don’t have experience with PPM encoder.