Declare Dependencies and Executables

Your Python files now exist. ROS still needs package/install metadata.

package.xml

Make sure runtime dependencies include:

<exec_depend>rclpy</exec_depend>
<exec_depend>std_msgs</exec_depend>

setup.py

Add console scripts:

entry_points={
    'console_scripts': [
        'hello = robotnix_pubsub.hello_node:main',
        'talker = robotnix_pubsub.talker:main',
        'listener = robotnix_pubsub.listener:main',
    ],
},

Read the path carefully

talker
  = robotnix_pubsub.talker : main
    └ module package/file    └ function

A typo here can build successfully and still make ros2 run unable to launch the executable correctly.

Resolve dependencies

From the workspace root:

cd ~/ros2_ws
rosdep install -i --from-path src --rosdistro jazzy -y

Checkpoint

For every executable, prove the target Python module and main() function actually exist.