I installed ROS 2 Galactic on my PC with Ubuntu Focal following the Ubuntu (Debian) Installation instruction.

I then installed turtlesim and called ros2 run turtlesim turtlesim_node and successfully open the simulation window.

I installed rqt and called rqt_graph and I can see the /turtlesim node and related topics such as /turtle1/cmd_vel.

Now, when I call ros2 node list, it returns nothing, and when I call ros2 topic list, there are only /parameter_events and /rosout.