Unit 3. Exercise 3.1 cant get info about topic

i am stuck

$ rostopic info /cmd_vel
ERROR: Unknown topic /cmd_vel
When i run
$ rostopic info | grep /cmd_vel
no output
Made log out-doesn’t help
What to do ?
How can i get info about this topic?

Did you run the commands given in the notebook before that? You should have run one of the following at some point.

source /opt/ros/noetic/setup.bash
# and/or
source ~/home/user/catkin_ws/devel/setup.bash

Please run these commands and try again.

yes i source environment, but nothing.
for some reason it start work now without me doing something
i think it some platform bug

Sorry about that. Next time, you can just load another course, then come back to this one, to “reset” the platform.