Turtlebot 3 ModuleNotFoundError: No module named 'SerialClient'

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I followed the tutorial but then when I run "roslaunch turtlebot3_bringup turtlebot3_robot.launch" I am getting this error

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SUMMARY
========

PARAMETERS
 * /rosdistro: melodic
 * /rosversion: 1.14.6
 * /turtlebot3_core/baud: 115200
 * /turtlebot3_core/port: /dev/ttyACM0
 * /turtlebot3_core/tf_prefix: 
 * /turtlebot3_lds/frame_id: base_scan
 * /turtlebot3_lds/port: /dev/ttyUSB0

NODES
  /
    turtlebot3_core (rosserial_python/serial_node.py)
    turtlebot3_diagnostics (turtlebot3_bringup/turtlebot3_diagnostics)
    turtlebot3_lds (hls_lfcd_lds_driver/hlds_laser_publisher)

ROS_MASTER_URI=http://192.168.1.184:11311

process[turtlebot3_core-1]: started with pid [4522]
ERROR: cannot launch node of type [hls_lfcd_lds_driver/hlds_laser_publisher]: hls_lfcd_lds_driver
ROS path [0]=/opt/ros/melodic/share/ros
ROS path [1]=/home/tonyu/catkin_ws/src
ROS path [2]=/opt/ros/melodic/share
process[turtlebot3_diagnostics-3]: started with pid [4523]
Traceback (most recent call last):
  File "/opt/ros/melodic/lib/rosserial_python/serial_node.py", line 39, in <module>
    from rosserial_python import SerialClient, RosSerialServer
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rosserial_python/__init__.py", line 1, in <module>
    from SerialClient import *

ModuleNotFoundError: No module named 'SerialClient'
[turtlebot3_core-1] process has died [pid 4522, exit code 1, cmd /opt/ros/melodic/lib/rosserial_python/serial_node.py __name:=turtlebot3_core __log:=/home/tonyu/.ros/log/65fcf586-c5e7-11ea-b958-0c8bfda0aa0a/turtlebot3_core-1.log].
log file: /home/tonyu/.ros/log/65fcf586-c5e7-11ea-b958-0c8bfda0aa0a/turtlebot3_core-1*.log