AttributeError: module 'genpy' has no attribute 'Message'

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Why i am getting this error:

Traceback (most recent call last):
  File "/home/asif/catkin_ws/src/beginner_tutorials/src/scripts/talker.py", line 39, in <module>
    import rospy
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rospy/__init__.py", line 47, in <module>
    from std_msgs.msg import Header
  File "/opt/ros/melodic/lib/python2.7/dist-packages/std_msgs/msg/__init__.py", line 1, in <module>
    from ._Bool import *
  File "/opt/ros/melodic/lib/python2.7/dist-packages/std_msgs/msg/_Bool.py", line 9, in <module>
    class Bool(genpy.Message):
AttributeError: module 'genpy' has no attribute 'Message'

when i try to run simple talker code on ros melodic installed in my nvidia jetson nano?