Failed to enable stream for custom mavlink message

Hi, I wanted to create a custom mavlink message (msg id = 325) and stream it. I followed the guide from this website [MAVLink Messaging | PX4 User Guide]. I added this line at the end of the rcS file.

mavlink stream -r 10 -s CUSTOM_GLOBAL_POSITION_INT -u 14550

My rcS file looks something like this

#!/bin/sh

set -e

# PX4 commands need the 'px4-' prefix in bash.
# (px4-alias.sh is expected to be in the PATH)
# shellcheck disable=SC1091
. px4-alias.sh

#search path for sourcing px4-rc.*
PATH="$PATH:${R}etc/init.d-posix"

#
# Main SITL startup script
#

# check for ekf2 replay
# shellcheck disable=SC2154
if [ "$replay_mode" = "ekf2" ]
then
	. ${R}etc/init.d-posix/rc.replay
	exit 0
fi

# initialize script variables
set IO_PRESENT                  no
set MIXER                       skip
set MIXER_AUX                   none
set MIXER_FILE                  none
set OUTPUT_MODE                 sim
set EXTRA_MIXER_MODE            none
set PWM_OUT                     none
set SDCARD_MIXERS_PATH          etc/mixers
set USE_IO                      no
set VEHICLE_TYPE                none
set LOGGER_ARGS                 ""
set LOGGER_BUF                  1000

set RUN_MINIMAL_SHELL           no

# Use the variable set by sitl_run.sh to choose the model settings.
if [ "$PX4_SIM_MODEL" = "shell" ]; then
	set RUN_MINIMAL_SHELL yes
else
	# Find the matching Autostart ID (file name has the form: [0-9]+_${PX4_SIM_MODEL})
	# TODO: unify with rc.autostart generation
	# shellcheck disable=SC2012
	REQUESTED_AUTOSTART=$(ls "${R}etc/init.d-posix/airframes" | sed -n 's/^\([0-9][0-9]*\)_'${PX4_SIM_MODEL}'$/\1/p')
	if [ -z "$REQUESTED_AUTOSTART" ]; then
		echo "ERROR  [init] Unknown model $PX4_SIM_MODEL (not found by name on ${R}etc/init.d-posix/airframes)"
		exit 1
	else
		echo "INFO  [init] found model autostart file as SYS_AUTOSTART=$REQUESTED_AUTOSTART"
	fi
fi

# Load parameters
set PARAM_FILE eeprom/parameters_"$REQUESTED_AUTOSTART"
param select $PARAM_FILE

if [ -f $PARAM_FILE ]
then
	if param load
	then
		echo "[param] Loaded: $PARAM_FILE"
	else
		echo "[param] FAILED loading $PARAM_FILE"
	fi
else
	echo "[param] parameter file not found, creating $PARAM_FILE"
fi

# exit early when the minimal shell is requested
[ $RUN_MINIMAL_SHELL = yes ] && exit 0


# Use environment variable PX4_ESTIMATOR to choose estimator.
if [ "$PX4_ESTIMATOR" = "q" ]; then
	param set SYS_MC_EST_GROUP 3
elif [ "$PX4_ESTIMATOR" = "ekf2" ]; then
	param set SYS_MC_EST_GROUP 2
elif [ "$PX4_ESTIMATOR" = "lpe" ]; then
	param set SYS_MC_EST_GROUP 1
elif [ "$PX4_ESTIMATOR" = "inav" ]; then
	param set SYS_MC_EST_GROUP 0
fi

if param compare SYS_AUTOSTART $REQUESTED_AUTOSTART
then
	set AUTOCNF no
else
	set AUTOCNF yes
	param set SYS_AUTOCONFIG 1
fi

if param compare SYS_AUTOCONFIG 1
then
	# Reset params except Airframe, RC calibration, sensor calibration, flight modes, total flight time, and next flight UUID.
	param reset_all SYS_AUTOSTART RC* CAL_* COM_FLTMODE* LND_FLIGHT* TC_* COM_FLIGHT*
	set AUTOCNF yes
fi

# multi-instance setup
# shellcheck disable=SC2154
param set MAV_SYS_ID $((px4_instance+1))

if [ $AUTOCNF = yes ]
then
	param set SYS_AUTOSTART $REQUESTED_AUTOSTART

	param set CAL_ACC0_ID  1310988 # 1310988: DRV_IMU_DEVTYPE_SIM, BUS: 1, ADDR: 1, TYPE: SIMULATION
	param set CAL_GYRO0_ID 1310988 # 1310988: DRV_IMU_DEVTYPE_SIM, BUS: 1, ADDR: 1, TYPE: SIMULATION
	param set CAL_ACC1_ID  1310996 # 1310996: DRV_IMU_DEVTYPE_SIM, BUS: 2, ADDR: 1, TYPE: SIMULATION
	param set CAL_GYRO1_ID 1310996 # 1310996: DRV_IMU_DEVTYPE_SIM, BUS: 2, ADDR: 1, TYPE: SIMULATION
	param set CAL_ACC2_ID  1311004 # 1311004: DRV_IMU_DEVTYPE_SIM, BUS: 3, ADDR: 1, TYPE: SIMULATION
	param set CAL_GYRO2_ID 1311004 # 1311004: DRV_IMU_DEVTYPE_SIM, BUS: 3, ADDR: 1, TYPE: SIMULATION

	param set CAL_MAG0_ID 197388
	param set CAL_MAG1_ID 197644

	param set SENS_BOARD_X_OFF 0.000001
	param set SENS_DPRES_OFF 0.001
fi

param set-default BAT1_N_CELLS 4

param set-default CBRK_AIRSPD_CHK 0
param set-default CBRK_SUPPLY_CHK 894281

# disable check, no CPU load reported on posix yet
param set-default COM_CPU_MAX -1

# Don't require RC calibration and configuration
param set-default COM_RC_IN_MODE 1

# Speedup SITL startup
param set-default EKF2_REQ_GPS_H 0.5

# Multi-EKF
param set-default EKF2_MULTI_IMU 3
param set-default SENS_IMU_MODE 0
param set-default EKF2_MULTI_MAG 2
param set-default SENS_MAG_MODE 0

param set-default IMU_GYRO_FFT_EN 1

param set-default -s MC_AT_EN 1

# By default log from boot until first disarm.
param set-default SDLOG_MODE 1
# enable default, estimator replay and vision/avoidance logging profiles
param set-default SDLOG_PROFILE 131
param set-default SDLOG_DIRS_MAX 7

param set-default TRIG_INTERFACE 3

param set-default SYS_FAILURE_EN 1

# Adapt timeout parameters if simulation runs faster or slower than realtime.
if [ -n "$PX4_SIM_SPEED_FACTOR" ]; then
	COM_DL_LOSS_T_LONGER=$(echo "$PX4_SIM_SPEED_FACTOR * 10" | bc)
	echo "COM_DL_LOSS_T set to $COM_DL_LOSS_T_LONGER"
	param set COM_DL_LOSS_T $COM_DL_LOSS_T_LONGER

	COM_RC_LOSS_T_LONGER=$(echo "$PX4_SIM_SPEED_FACTOR * 0.5" | bc)
	echo "COM_RC_LOSS_T set to $COM_RC_LOSS_T_LONGER"
	param set COM_RC_LOSS_T $COM_RC_LOSS_T_LONGER

	COM_OF_LOSS_T_LONGER=$(echo "$PX4_SIM_SPEED_FACTOR * 0.5" | bc)
	echo "COM_OF_LOSS_T set to $COM_OF_LOSS_T_LONGER"
	param set COM_OF_LOSS_T $COM_OF_LOSS_T_LONGER

	COM_OBC_LOSS_T_LONGER=$(echo "$PX4_SIM_SPEED_FACTOR * 5.0" | bc)
	echo "COM_OBC_LOSS_T set to $COM_OBC_LOSS_T_LONGER"
	param set COM_OBC_LOSS_T $COM_OBC_LOSS_T_LONGER
fi

# Autostart ID
autostart_file=''
# shellcheck disable=SC2231
for f in ${R}etc/init.d-posix/airframes/"$(param show -q SYS_AUTOSTART)"_*
do
	filename=$(basename "$f")
	case "$filename" in
		*\.*)
			# ignore files that contain a dot (e.g. <vehicle>.post)
			;;
		*)
			autostart_file="$f"
			;;
	esac
done
if [ ! -e "$autostart_file" ]; then
	echo "Error: no autostart file found ($autostart_file)"
	exit 1
fi

. "$autostart_file"

# Simulator IMU data provided at 250 Hz
param set IMU_INTEG_RATE 250

#user defined params for instances can be in PATH
. px4-rc.params

dataman start
# start sih in sih_sim mode, otherwise simulator module
if [ "$SIM_MODE" = "sihsim" ]; then
	sih start
# only start the simulator if not in replay mode, as both control the lockstep time
elif ! replay tryapplyparams
then
  . px4-rc.simulator
fi
load_mon start
battery_simulator start
tone_alarm start
rc_update start
manual_control start
sensors start
commander start

# Configure vehicle type specific parameters.
# Note: rc.vehicle_setup is the entry point for rc.interface,
#       rc.fw_apps, rc.mc_apps, rc.rover_apps, and rc.vtol_apps.
#
. ${R}etc/init.d/rc.vehicle_setup

navigator start

# Try to start the micrortps_client with UDP transport if module exists
if px4-micrortps_client status > /dev/null 2>&1
then
	. px4-rc.rtps
fi

if param greater -s MNT_MODE_IN -1
then
	gimbal start
fi

if param greater -s TRIG_MODE 0
then
	camera_trigger start
	camera_feedback start
fi

if param compare -s IMU_GYRO_FFT_EN 1
then
	gyro_fft start
fi

if param compare -s IMU_GYRO_CAL_EN 1
then
	gyro_calibration start
fi

#user defined mavlink streams for instances can be in PATH
. px4-rc.mavlink

# execute autostart post script if any
[ -e "$autostart_file".post ] && . "$autostart_file".post

# Run script to start logging
if param compare SYS_MC_EST_GROUP 2
then
	set LOGGER_ARGS "-p ekf2_timestamps"
else
	set LOGGER_ARGS "-p vehicle_attitude"
fi
. ${R}etc/init.d/rc.logging

mavlink boot_complete
replay trystart

mavlink stream -r 10 -s CUSTOM_GLOBAL_POSITION_INT -u 14550

However, when I try to run px4, it says

WARN  [mavlink] mavlink for network on port 14550 is not running
ERROR [px4] Startup script returned with return value: 256
recvfrom error: Connection reset by peer
Connection closed by client.
Connection closed by client.
Connection closed by client.