Hi,
Are you working in the simulation? The reason why you might not see the cmd_vel topic is because no one is publishing to the topic yet, so the bridge connection hasn’t been made.
As for the teleop issue, sometimes the sourcing isn’t done right when you past more than one command at a time. Try typing them one by one in a terminal:
source ros2_ws/install/setup.bashexport TURTLEBOT3_MODEL=burgerros2 run turtlebot3_teleop teleop_keyboard