ModuleNotFoundError: No module named 'error'

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Hi, I am a beginner in Python, Ros and Ubuntu and am having difficulties in running the first program on ROS, i.e. listener.py I am running Ros melodic in Ubuntu 18.04 The following error report comes up on running "rosrun hello_world listener.py"

joseph@joseph-Inspiron-5558:~$ rosrun hello_world talker.py 
Traceback (most recent call last):
  File "/home/joseph/catkin_ws/src/hello_world/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 5, in <module>
    import genpy
  File "/opt/ros/melodic/lib/python2.7/dist-packages/genpy/__init__.py", line 34, in <module>
    from . message import Message, SerializationError, DeserializationError, MessageException, struct_I
  File "/opt/ros/melodic/lib/python2.7/dist-packages/genpy/message.py", line 44, in <module>
    import yaml
  File "/usr/lib/python2.7/dist-packages/yaml/__init__.py", line 2, in <module>
    from error import *
**ModuleNotFoundError: No module named 'error'**
Error in sys.excepthook:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 53, in apport_excepthook
    if not enabled():
  File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 24, in enabled
    import re
  File "/usr/lib/python3.6/re.py", line 142, in <module>
    class RegexFlag(enum.IntFlag):
**AttributeError: module 'enum' has no attribute 'IntFlag'**

Original exception was:
Traceback (most recent call last):
  File "/home/joseph/catkin_ws/src/hello_world/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 5, in <module>
    import genpy
  File "/opt/ros/melodic/lib/python2.7/dist-packages/genpy/__init__.py", line 34, in <module>
    from . message import Message, SerializationError, DeserializationError, MessageException, struct_I
  File "/opt/ros/melodic/lib/python2.7/dist-packages/genpy/message.py", line 44, in <module>
    import yaml
  File "/usr/lib/python2.7/dist-packages/yaml/__init__.py", line 2, in <module>
    from error import *
**ModuleNotFoundError: No module named 'error'**

The text in bold are the error messages and they seem to be something related to the version conflict in the multiple installations of python.

Any help in resolving this matter would be appreciated Thanks.