XML syntax error while running a launch file

edit

I am using ROS melodic on ubuntu 18.04. I was trying to run the following launch file:

<launch>
    <param name="teleop_topic" value="/turtle1/cmd_vel"/>
    <!-- Loading teleop parameter file -->
    <rosparam command="load" file="$(find gesture_teleop)/config/teleop_config.yaml"/>
    <!-- Launching ros serial nodes -->
    <node name="rosserial_server_node" pkg="rosserial_python" type="serial_node.py" args="$(arg port)" output="screen"/>
    <!-- Starting gesture teleop node -->
    <node name="gesture_teleop_node" pkg="gesture_teleop" type="gesture_teleop.py" output="screen"/>
    <!-- Starting turtlebot 2D simualation -->
    <node name="turtlebot_node" pkg="turtlesim" type="turtlesim_node" output="screen"/>
</launch>

However , it gave an error:

RLException: Invalid roslaunch XML syntax: no element found: line 1, column 0
The traceback for the exception was written to the log file

What exactly is the error in the first line?