Eventually, I have found the solution, but first I will tell the exact symptoms: α) no activity lights on the Raspberry Pi, b) no activity to the Raspberry Pi’s port, c) the starting sounds of Pixhawk’s startup were playing but very very very low.
At first, I thought that I have no power to the Raspberry Pi so I changed the UBEC but nothing, second I checked every cable about the power supply of Raspberry Pi but the result was zero.
I contacted with a professional builder of drones using Pixhawk and he advised me to check the power module of Pixhawk and…when I unplugged/plugged the data connector between Pixhawk and power module the problem was solved and the BR started up with no problem.
I don’t know if somebody else had the same problem but I write it in any case.
@jwalser have you any reference about Pixhawk’s power module failure? Is there anything that can we do for precautionary reasons?
Thank everyone for the advises…