Missing joint_state_publisher_gui when l run display.launch

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A recent PR (ros/joint_state_publisher#31) split the GUI parts of the JSP from the non-GUI parts.

This was done to avoid depending on GUI libraries in the non-GUI ROS metapackages (those defined in REP-142 and REP-150). This facilitates deployments of ROS on robots, as it avoids pulling in all sorts of GUI libraries when they are not needed.

To fix the error you would have to make sure to install the joint_state_publisher_gui package. On Ubuntu/Debian (and other OS for which .deb packages are available and apt is used), you could do that with the following command:

sudo apt update
sudo apt install ros-<your_ros_version>-joint-state-publisher-gui

Make sure to replace <your_ros_version> with the code name of the ROS version you are using. So for Kinetic, replace it with kinetic. For Melodic, replace it with melodic.

The first command is to make sure you've downloaded up-to-date package indices.

The second command actually installs the package.