lidar sensor don't read data in gazebo

edit

I could not read lidar sensor data in gazebo

Is there a topic scan when running rostopic list at least ?

blue area scanned by lidar is not visible.

Are the <visualize> and <always_on> set to true in the sdf file ?

You need to give more informations like the nodes/launch files you are launching, the output of the terminal if there is an error etc...