SyntaxError: invalid syntax during launching move_it

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Hey i'm new in ROS. I've been tried to launch moveit! roslaunch moveit_setup_assistant setup_assistant.launch i'd got these error "SyntaxError: invalid syntax ". These are my traceback:

Traceback (most recent call last):
  File "/opt/ros/melodic/bin/roslaunch", line 34, in <module>
    import roslaunch
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roslaunch/__init__.py", line 62, in <module>
    from .scriptapi import ROSLaunch
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roslaunch/scriptapi.py", line 42, in <module>
    import roslaunch.parent
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roslaunch/parent.py", line 54, in <module>
    import roslaunch.server
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roslaunch/server.py", line 79, in <module>
    from rosgraph_msgs.msg import Log
  File "/opt/ros/dashing/lib/python3.6/site-packages/rosgraph_msgs/msg/__init__.py", line 1, in <module>
    from rosgraph_msgs.msg._clock import Clock  # noqa: F401
  File "/opt/ros/dashing/lib/python3.6/site-packages/rosgraph_msgs/msg/_clock.py", line 56
    class Clock(metaclass = Metaclass_Clock):
                          ^
SyntaxError: invalid syntax

And these might be related error but i've feel doesn't help :https://answers.ros.org/question/221035/ros-scripts-and-python-related-error/ Please I need help, and anybody have suggestion ?