I am not sure if this is the right place to report this, but I am not able to follow the steps described here with Dashing installed:

https://ros-planning.github.io/navigation2/getting_started/index.html

Apparently, the release of Dashing of nav2_bringup does not contain the file tb3_simulation_launch.py.

https://github.com/ros-planning/navigation2/tree/dashing-devel/nav2_bringup