RLException: neither a launch file package nor a launch file name

Question

To get straight to the point I want to know how to fix this error and understand it, what is an RLException? Why am I getting this? How can I avoid this later? Below I will post what I did step by step:

This is the directory where I have my files

~/Desktop/Research/research_ws

This is the error i'm getting:

RLException: [ddrobot_rviz.launch] is neither a launch file in package [ros_robotics] nor is [ros_robotics] a launch file name
The traceback for the exception was written to the log file

The following are the codes for dd_robot.urdf and ddrobot_rviz.launch respectively:

<robot name="dd_robot">
    
    <!-- Base Link -->
    <link name ="base_link">
        <visual>
            <origin xyz = "0 0 0" rpy = "0 0 0" />
            <geometry>
                <box size = ".5 .5 .25"/>
            </geometry>
        </visual>
    </link>
</robot>

    <!-- values passed by command line input -->
    <arg name = "model" />
    <arg name = "gui" default ="False" />
    
    <!-- Set these parameters on Parameter Server -->
    <param name = "robot_description"
        textfile = "$(find ros_robotics)/urdf/$(arg model)" />
    <param name = "use_gui" value = "$(arg gui)" />
    
    <!-- Start 3 nodes: joint_state_publisher, robot_state_publisher
    rviz -->
    <node name = "joint_state_publisher" pkg = "joint_state_publisher"
            type = "joint_state_publisher" />
    <node name = "robot_state_publisher" pkg = "robot_state_publisher"
            type = "state_publisher" />
    <node name = "rviz" pkg = "rviz" type = "rviz" 
        args = "-d $(find ros_robotics)/urdf.rviz"
        required = "true" />
</launch>

I do not yet completely understand what is written in the above codes, I'm following the ROS robotics by example book Fairchild.

Answer 1

I just had to re-enter:

source devel/setup.bash

In every new terminal.