No rc receiver and unable to switch flight modes

I have recently set up pixhawk 2.1 with a fbl unit downstream. I get the message “no rc receiver” in the HUD when the transmitter is turned on. When I turn the transmitter off it goes away. I can see full range in the transmitter calibration as well as in the fbl unit set up. I am also unable to switch flight modes. I can see it switch on the flight mode page but it does not show the flight modes changing in the HUD. What is the remedy?

You get that with Taranis TX and RX when they are to close to each other. Can happen mostly during bench testing.