Cannot spawn a .xacro model using a launch file

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Hello. I'm very new to ROS & gazebo.

After I run the launch file the terminal gets stuck with: [INFO] [1595010136.688080, 0.000000]: Waiting for service /gazebo/spawn_urdf_model

The launch file:

<launch>

<!-- These are the arguments you can pass to this launch file, for example paused:=true -->
<arg name="paused" default="false"/>
<arg name="use_sim_time" default="true"/>
<arg name="gui" default="true"/>
<arg name="headless" default="false"/>
<arg name="debug" default="false"/>

<!-- We resume the logic in empty_world.launch -->
<include file="$(find gazebo_ros)/launch/empty_world.launch">
    <arg name="debug" value="$(arg debug)" />
    <arg name="gui" value="$(arg gui)" />
    <arg name="paused" value="$(arg paused)" />
    <arg name="use_sim_time" value="$(arg use_sim_time)" />
    <arg name="headless" value="$(arg headless)" />
</include>

<!-- Load the URDF into the ROS parameter server -->
<param name="robot_description" command="$(find xacro)/xacro --inorder '$(find mastering_ros_robot_description_pkg)/urdf/seven_dof_arm.xacro'" />

<!-- Run a python script to send a service call to gazebo_ros to spawn a URDF robot-->
<node name="urdf_spawner" pkg="gazebo_ros" type="spawn_model" respawn="false" output="screen" args="-urdf -model seven_dof_arm -param robot_description"/>

</launch>

Basically this is the package I am trying to replicate as a way of practicing ROS/gazebo: link

In addition, I get this error (?): [gazebo-2] process has died [pid 27516, exit code 255, cmd /opt/ros/kinetic/lib/gazebo_ros/gzserver -e ode worlds/empty.world __name:=gazebo __log:=/home/ami/.ros/log/6e5a50b4-c85a-11ea-abae-0800279c7921/gazebo-2.log]. log file: /home/ami/.ros/log/6e5a50b4-c85a-11ea-abae-0800279c7921/gazebo-2*.log

Not sure if it's related to the issue of spawning the model.

Would really appreciate any help with trying to figure out the problem. Thank you in advance.