Unable to use ros2 after installing using debian-packages

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I have installed ROS 2 (ros-dashing-desktop) via debian packages as per the following instructions:

https://index.ros.org/doc/ros2/Instal...

I am using Ubuntu 18.04.

After installation, I source the ros-dashing workspace using the following command:

 source /opt/ros/dashing/setup.bash

When I try to run the ros 2 demo example using the following command:

ros2 run demo_nodes_cpp talker

I get the following error:

ros2 run demo_nodes_cpp talker Failed to load entry point 'security': No module named 'ament_index_python' Failed to load entry point 'run': No module named 'ament_index_python' Failed to load entry point 'launch': No module named 'ament_index_python' Failed to load entry point 'info': No module named 'rclpy' Failed to load entry point 'list': No module named 'ament_index_python' Failed to load entry point 'send_goal': No module named 'action_msgs' Failed to load entry point 'show': No module named 'ament_index_python' Failed to load entry point 'list': No module named 'rclpy' Failed to load entry point 'load': No module named 'rclpy' Failed to load entry point 'standalone': No module named 'rclpy' Failed to load entry point 'types': No module named 'rclpy' Failed to load entry point 'unload': No module named 'rclpy' Failed to load entry point 'start': No module named 'rclpy' Failed to load entry point 'status': No module named 'rclpy' Failed to load entry point 'stop': No module named 'rclpy' Failed to load entry point 'get': No module named 'rclpy' Failed to load entry point 'list': No module named 'rclpy' Failed to load entry point 'nodes': No module named 'rclpy' Failed to load entry point 'set': No module named 'rclpy' Failed to load entry point 'list': No module named 'ament_index_python' Failed to load entry point 'package': No module named 'ament_index_python' Failed to load entry point 'packages': No module named 'ament_index_python' Failed to load entry point 'show': No module named 'ament_index_python' Failed to load entry point 'info': No module named 'rclpy' Failed to load entry point 'list': No module named 'rclpy' Failed to load entry point 'delete': No module named 'rcl_interfaces' Failed to load entry point 'get': No module named 'rcl_interfaces' Failed to load entry point 'list': No module named 'rcl_interfaces' Failed to load entry point 'set': No module named 'rcl_interfaces' Failed to load entry point 'create': No module named 'ament_index_python' Failed to load entry point 'executables': No module named 'ament_index_python' Failed to load entry point 'list': No module named 'ament_index_python' Failed to load entry point 'prefix': No module named 'ament_index_python' Failed to load entry point 'call': No module named 'rclpy' Failed to load entry point 'list': No module named 'rclpy' Failed to load entry point 'list': No module named 'ament_index_python' Failed to load entry point 'package': No module named 'ament_index_python' Failed to load entry point 'packages': No module named 'ament_index_python' Failed to load entry point 'show': No module named 'ament_index_python' Failed to load entry point 'bw': No module named 'rclpy' Failed to load entry point 'delay': No module named 'rclpy' Failed to load entry point 'echo': No module named 'rclpy' Failed to load entry point 'hz': No module named ...