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It does say at the top of the tutorial that it is incomplete:

it is likely outdated and should not be expected to compile as is

Also, the tutorials repository is not being actively updated. Refer to the examples and demos packages instead:

• https://github.com/ros2/examples
• https://github.com/ros2/demos

For an example of building a new message, you can check out the example_interfaces package: https://github.com/ros2/example_interfaces

Or look at the action tutorial: https://index.ros.org/doc/ros2/Tutorials/Actions/