ERROR: can't locate node in package [gazebo_ros]

Hello~ I'm new in ROS and am learning it by the tutorials.However,there were some errors when I followed the [Tutorial: Using a URDF in Gazebo](http://gazebosim.org/tutorials/?tut=ros_urdf). Everthing is normal until I ran the the command: roslaunch rrbot_gazebo rrbot_world.launch  However, it seems I can start gazebo by command line.  Thanks a lot for your help!