Robot simulation wheels spinning on the spot malfunction

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My URDF is loading in Gazebo and I issue a twist command to move the robot along the x-axis.

The problem is the wheels malfunction by spinning instead of rotating and moving the robot as shown in the image below:

I would appreciate some help with this.

My urdf code is below:

<?xml version="1.0"?>
<robot name="mybot">

  <gazebo>
    <plugin name="differential_drive_controller" filename="libgazebo_ros_diff_drive.so">
      <legacyMode>false</legacyMode>
      <alwaysOn>true</alwaysOn>
      <publishWheelTF>true</publishWheelTF>
      <publishTf>1</publishTf>
      <publishWheelJointState>true</publishWheelJointState>
      <updateRate>100.0</updateRate>
      <leftJoint>left_wheel_joint</leftJoint>
      <rightJoint>right_wheel_joint</rightJoint>
      <wheelSeparation>1.1</wheelSeparation>
      <wheelDiameter>0.52</wheelDiameter>
      <wheelAcceleration>1.0</wheelAcceleration>
      <torque>20</torque>
      <commandTopic>cmd_vel</commandTopic>
      <odometryTopic>odom</odometryTopic>
      <odometryFrame>odom</odometryFrame>
      <robotBaseFrame>base_link</robotBaseFrame>
    </plugin>
  </gazebo>

  <material name="blue">
    <color rgba="0 0 0.8 1"/>
  </material>
  <material name="black">
    <color rgba="0 0 0 1"/>
  </material>
  <material name="white">
    <color rgba="1 1 1 1"/>
  </material>
  <material name="red">
    <color rgba="1 0 0 1"/>
  </material>

  <link name="base_link">
    <visual>
      <geometry>
        <cylinder length="0.015" radius="0.2"/>
      </geometry>
      <material name="white"/>
    </visual>
    <collision>
      <geometry>
        <cylinder length="0.015" radius="0.2"/>
      </geometry>
      <material name="white"/>
    </collision>
    <inertial>
      <origin xyz="0 0 1" rpy="0 0 0"/>
      <mass value="1"/>
      <inertia
        ixx="1.0" ixy="0.0" ixz="0.0"
        iyy="1.0" iyz="0.0"
        izz="1.0"/>
    </inertial>
  </link>

        <link name="upper_level">
          <visual>
            <geometry>
              <cylinder length="0.015" radius="0.2"/>
            </geometry>
            <material name="white"/>
          </visual>
          <collision>
            <geometry>
              <cylinder length="0.015" radius="0.2"/>
            </geometry>
          </collision>
          <inertial>
            <origin xyz="0 0 1" rpy="0 0 0"/>
            <mass value="1"/>
            <inertia
              ixx="1.0" ixy="0.0" ixz="0.0"
              iyy="1.0" iyz="0.0"
              izz="1.0"/>
          </inertial>
        </link>
        <joint name='upper_level_joint' type='fixed'>
          <parent link='base_link'/>
          <child link='upper_level'/>
          <origin rpy='0 0 0' xyz='0 0 0.05'/>
        </joint>

        <link name='seperator_1'>
          <visual>
             <origin rpy='0 0 0' xyz='-0.120 0.085 -0.02'/>
             <material name='white'/>
            <geometry>
              <cylinder length='0.07' radius='0.003'/>
            </geometry>
          </visual>
          <collision>
             <material name='white'/>
            <geometry>
              <cylinder length='0.07' radius='0.003'/>
            </geometry>
          </collision>
          <inertial>
            <origin xyz="0 0 1" rpy="0 0 0"/>
            <mass value="1"/>
            <inertia
              ixx="1.0" ixy="0.0" ixz="0.0"
              iyy="1.0" iyz="0.0"
              izz="1.0"/>
          </inertial>
        </link>
        <joint name='upper_level_sperator_1_joint' type='fixed'>
          <parent link='upper_level'/>
          <child link='seperator_1'/>
          <origin xyz='0 0 0'/>
        </joint>

        <link name='seperator_2'>
          <visual>
             <origin rpy='0 0 0' xyz='-0.120 -0.085 -0.02'/>
             <material name='white'/>
            <geometry>
              <cylinder length='0.07' radius='0.003'/>
            </geometry>
          </visual>
          <collision>
            <geometry>
              <cylinder length='0.07' radius='0.003'/>
            </geometry>
          </collision>
          <inertial>
            <origin xyz="0 0 1" rpy="0 0 0"/>
            <mass value="1"/>
            <inertia
              ixx="1.0" ixy="0.0" ixz="0.0"
              iyy="1.0" iyz="0.0"
              izz="1.0"/>
          </inertial>
        </link>
        <joint name='upper_level_sperator_2_joint' type='fixed'>
          <parent link='upper_level'/>
          <child link='seperator_2'/>
          <origin xyz='0 0 0'/>
        </joint>

        <link name='seperator_3'>
          <visual>
             <origin ...