Pixhawk not able to ARM without connecting cable to QGC on laptop

Hello, I have recently built a Pixhawk based drone. I have used QGroundControl software to flash the firm ware and setup the drone. All the sensors and calibrated including the power, radio and other. Everything works absolutely fine on check.

But, when I unplug the cable from Pixhawk to the Laptop where QGC is running and then try to ARM the drone it doesn’t ARM. I don’t understand why this happens??

Can someone please kindly help me solve this issue. It would be very helpful.