no signal HEARTBEAT using serial conneciton on companion computer

[yncity]

from pymavlink import mavutil

 Start a connection listening on a UDP port
the_connection = mavutil.mavlink_connection('/dev/ttyUSB0')

# Wait for the first heartbeat
#   This sets the system and component ID of remote system for the link
print('waiting for heartbeat...')
the_connection.wait_heartbeat(timeout=10)
print("Heartbeat from system (system %u component %u)"%(the_connection.target_system, the_connection.target_component))

I use this code to check my connection Pixhawk from Pi 4B, but it doesn't work.

So I found some solutions but it also doesn't work.

#372 (comment)

https://discuss.ardupilot.org/t/unable-to-establish-a-connection-to-sitl-pixhawk-via-udp-tcp-serial/85922/2

https://arc.net/l/quote/eklxslrh

how can I solve this problem?
when I use 'mavproxy.py' it works well.

I install Ubuntu24.04 on Pi (machine = aarch64)
and my pymavlink version

Name: pymavlink
Version: 2.4.41
Summary: Python MAVLink code
Home-page: https://github.com/ArduPilot/pymavlink/
Author: 
Author-email: 
License: LGPLv3
Location: /home/ioes/venv-ardupilot/lib/python3.12/site-packages
Requires: future, lxml
Required-by: dronekit, MAVProxy