This robot has a joint named "wheel_right_joint" which is not in the gazebo model.

edit

Joint tag does not belong inside the transmission tag.

You define your joints in model tags. Then you pass the transmission tags only the name of the joint.

<joint name="wheel_right_joint" type="revolute">
      <insert_block name="origin" />
      <limit effort="1000.0" lower="-1e+16" upper="1e+16" velocity="3.5"/>
      <axis xyz="-1 0 0" />
      <joint_properties damping="0" friction="0.0" />
      <parent link="base_link" />
      <child link="${name}_link" />
</joint>

<transmission name="tran_${number}">
    <type>transmission_interface/SimpleTransmission</type>
    <joint name="wheel_right_joint">
        <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
    </joint>
    <actuator name="left_wheel_actuator">
        <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
        <mechanicalReduction>7</mechanicalReduction>
    </actuator>
</transmission>