Set_rc_channel_pwm() not working

Hello

I’m working on the AUV. I am using Raspberry pi 3 B+ and pixhawk 4. I am trying the below code to start the engines but nothing is working. Can you help me with this?

from pymavlink import mavutil
import time

master=mavutil.mavlink_connection(’/dev/ttyACM0’,baund=115200)

master.wait_heartbeat()

def set_rc_channel_pwm(id,pwm=1500):

if id < 1:
    print("Channel does not exist.")
    return

if id < 9:
    rc_channel_values = [65535 for _ in range(8)]
    rc_channel_values[id - 1] = pwm

    master.mav.rc_channels_override_send(master.target_system,master.target_component,*rc_channel_values)                  

master.mav.command_long_send(master.target_system,master.target_component,mavutil.mavlink.MAV_CMD_COMPONENT_ARM_DISARM,0,1,0,0,0,0,0,0)

master.motors_armed_wait()

while True:

set_rc_channel_pwm(4,1600)