Could not contact the following nodes: * /joint_state_publisher

edit

Finally think that I solved the problem.

I forgot to add in my launch file the correct path for the joint_state_publisher and set the gui parameter to true.

Finally, my launch file looks like this:

<launch>

    <param name="robot_description" textfile="/opt/ros/cturtle/stacks/urdf_tutorial/06-flexible.urdf" />
    <param name="use_gui" value="true"/> 
    <node name="robot_state_publisher" pkg="robot_state_publisher" type="state_publisher" />
    <node name="joint_state_publisher" pkg="joint_state_publisher" type="joint_state_publisher" />
    <node name="rviz" pkg="rviz" type="rviz" />

</launch>