About the "[gazebo_gui-3] process has died"

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@1210652636 Gazebo is a simulation program that comes with two entities: gzserver that is the core of the simulation and gzclient that is the GUI. What is failing in your machine is the GUI. So it may be something realted with your graphical set up. Have you tried to run Gazebo without GUI and see what happens? Also, ensure the world you are using exists and the environmental variables for the turtlebot are properly set up.

Could you tell me how to set up the gzclient? I don't recall that word ever appearing during my installation.

@1210652636 you do not really need to set up the gzclient since it should work with any problems. For the moment you only want to know what is the problem. For this reason, to debug this, first, you can launch Gazebo alone and see if it is something related with your machine or with the launch files. Open a terminal and launch Gazebo (E.g.: gazebo empty.wold). Then if the same error appears try to launch Gazebo without the client, setting the parameter of the launch file to false. (E.g.: <arg name="gui" value="false"/>). If the error is still going then it must be something related with your graphics.

Thank you so much I will try it.

By the way, when I open a new terminal and run the code "export SVGA_VGPU10=0", this error is gone. But next time, when I run "roslaunch turtlebot3_gazebo turtlebot3_empty_world.launch", the same error happened. So I have to run "export SVGA_VGPU10=0" again and again befor run "roslaunch ... ".