So I just executed it and it worked. So it must have been some isue related to have opened the Noetic course yesterday. We deativated them due to some issues that were detected.
Could you do the following:
remove all the buidl an ddevel folders of your catkin_ws and recmpile it like so:
rm -rf build/ devel/
source devel/setup.bash ;rospack profile
Now you should be able to do roslaunch, rostopic or whatever commands of ros.
Now just change to a unit of the course that changes the simulation ( Unit8 for example)
And return back to this Unit9.
The simulation should launch correctly.
If not please tell us