Fatal - pan_tilt_port: No motors found

[ycho80]

Ubuntu 16.04 Kinetic
I am following Connecting to Dynamixel Bus tutorial and I am using one XM dynamixel motor but I am getting error like
[FATAL] [1566659793.371607]: pan_tilt_port: No motors found.
================================================================================REQUIRED process [dynamixel_manager-2] has died!
process has died [pid 28029, exit code 1, cmd /opt/ros/kinetic/lib/dynamixel_controllers/controller_manager.py __name:=dynamixel_manager __log:=/home/cho/.ros/log/261b67fa-c682-11e9-a3d5-5ce0c5381666/dynamixel_manager-2.log].
log file: /home/cho/.ros/log/261b67fa-c682-11e9-a3d5-5ce0c5381666/dynamixel_manager-2*.log
Initiating shutdown!

Please give me an advice how to fix this issue.

my launch file is

<launch>
    <node name="dynamixel_manager" pkg="dynamixel_controllers" type="controller_manager.py" required="true" output="screen">
        <rosparam>
            namespace: dxl_manager
            serial_ports:
                pan_tilt_port:
                    port_name: "/dev/ttyUSB0"
                    baud_rate: 57600
                    min_motor_id: 1
                    max_motor_id: 25
                    update_rate: 20
        </rosparam>
    </node>
</launch>

CM file is

/cmake_minimum_required(VERSION 2.8.3)
project(my_dynamixel_tutorial)
find_package(catkin REQUIRED)

catkin_package(
    CATKIN_DEPENDS dynamixel_controllers
)

install(DIRECTORY launch
  DESTINATION ${CATKIN_PACKAGE_SHARE_DESTINATION}
  )

install(DIRECTORY config
  DESTINATION ${CATKIN_PACKAGE_SHARE_DESTINATION}
  )

Package file is

<package>
  <name>my_dynamixel_tutorial</name>
  <version>0.4.1</version>
  <description>
    Example configuration and launch file for dynamixel_motor stack.
  </description>
  <maintainer email="arebgun@gmail.com">Antons Rebguns</maintainer>

  <license>BSD</license>

  <url type="website">http://ros.org/wiki/dynamixel_driver</url>
  <url type="bugtracker">https://github.com/arebgun/dynamixel_motor</url>

  <author>Antons Rebguns</author>

  <buildtool_depend>catkin</buildtool_depend>
  <run_depend>dynamixel_controllers</run_depend>

  <export>

  </export>
</package>