I'm new to ROS. I'm doing a project on differential drive rover in Ubuntu 20.04.5. I successfully simulated the gazebo and rviz files but I'm facing two main issues: * In Rviz, all the options are displayed in fixed frame except the odom option while I have simulated my previous project with the same launch file for Rviz which I am using for this project. * The second issue is that when I enter rosrun teleoptwistkeyboard teleoptwistkeyboard.py in the command window, I get 'Wait for subscriber to connect to \cmdvel' even though there is a topic '\cmdvel' when I type rostopic list. How could I solve these problems?