Not able to subscribe to /cmd_vel topic

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Hi, I'm new to ros,I followed a tutorial in internet to build a two-wheeled robot.Here is the problem,when i run rosrun teleop_twist_keyboard teleop_twist_keyboard.py the output is

Waiting for subscriber to connect to /cmd_vel

here is launch file:

<?xml version="1.0" encoding="UTF-8"?>
<launch>
<param name="robot_description" command="$(find xacro)/xacro --inorder '$(find m2wr_description)/urdf/m2wr.xacro'" />

<arg name="x" default="0"/>
<arg name="y" default="0"/>
<arg name="z" default="0.5"/>

<node name="mybot_spawn" pkg="gazebo_ros" type="spawn_model" output="screen"
      args="-urdf -param robot_description -model m2wr -x $(arg x) -y $(arg y) -z $(arg z)" />

<node pkg="teleop_twist_keyboard" type="teleop_twist_keyboard.py" name="teleop">
</node>

 </launch>

Here is my gazebo plugin code:

  <gazebo>
<plugin filename="libgazebo_ros_diff_drive.so" name="differential_drive_controller">
  <alwaysOn>true</alwaysOn>
  <updateRate>20</updateRate>
  <leftJoint>joint_left_wheel</leftJoint>
  <rightJoint>joint_right_wheel</rightJoint>
  <wheelSeparation>0.4</wheelSeparation>
  <wheelDiameter>0.2</wheelDiameter>
  <torque>0.1</torque>
  <commandTopic>cmd_vel</commandTopic>
  <odometryTopic>odom</odometryTopic>
  <odometryFrame>odom</odometryFrame>
  <robotBaseFrame>link_chassis</robotBaseFrame>
</plugin>
</gazebo>

I don't know what is the problem. Please help!