Turtlebot4 Slam error "could not found transform from [] to map"
I am working on turtlebot4 and ros2 galactic. I facing issues in SLAM. Rviz first showed me "could not found transform from [] to map" then i changed the rviz config file then it displayed "could not found transfor from [] to base link" . The lidar.ranges data is also 0 and does not change even after putting obstacles infront of robot.