I'm following this tutorial: https://www.generationrobots.com/blog/en/2015/02/robotic-simulation-scenarios-with-gazebo-and-ros/

and get this when trying to launch:

timmx@timmx-NV52L:~/catkin_ws/src/mybot_description/urdf$ roslaunch  mybot_gazebo mybot_world.launch 
... logging to /home/timmx/.ros/log/f8a88710-7f35-11e8-80fd-20689d9ebc85/roslaunch-timmx-NV52L-27035.log
Checking log directory for disk usage. This may take awhile.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

xacro: Traditional processing is deprecated. Switch to --inorder processing!
To check for compatibility of your document, use option --check-order.
For more infos, see http://wiki.ros.org/xacro#Processing_Order
xacro:arg: missing attribute 'default'
when processing file: /home/timmx/catkin_ws/src/mybot_description/urdf/mybot.xacro
Invalid <param> tag: Cannot load command parameter [robot_description]: command [/opt/ros/kinetic/share/xacro/xacro.py '/home/timmx/catkin_ws/src/mybot_description/urdf/mybot.xacro'] returned with code [2]. 

Param xml is <param command="$(find xacro)/xacro.py '$(find mybot_description)/urdf/mybot.xacro'" name="robot_description"/>
The traceback for the exception was written to the log file

I have written the following files:

MYBOT.XACRO

<?xml version="1.0"?>
<robot name="mybot" xmlns:xacro="http://www.ros.org/wiki/xacro">
  <!-- First ROS-Gazebo robot -->

  <xacro:property name="PI" value="3.1415926535897931"/>
  <xacro:property name="chassisHeight" value="0.1"/>
  <xacro:property name="chassisLength" value="0.4"/>
  <xacro:property name="chassisWidth" value="0.2"/>
  <xacro:property name="chassisMass" value="50"/>

  <xacro:property name="casterRadius" value="0.05"/>
  <xacro:property name="casterMass" value="5"/>

  <xacro:property name="wheelWidth" value="0.05"/>
  <xacro:property name="wheelRadius" value="0.1"/>
  <xacro:property name="wheelPos" value="0.2"/>
  <xacro:property name="wheelMass" value="5"/>

  <xacro:property name="cameraSize" value="0.05"/>
  <xacro:property name="cameraMass" value="0.1"/>

  <xacro:arg name="gui" value="true"/>

  <xacro:include filename="$(find mybot_description)/urdf/mybot.gazebo" />
  <xacro:include filename="$(find mybot_description)/urdf/materials.xacro" />
  <xacro:include filename="$(find mybot_description)/urdf/macros.xacro" />

  <link name="footprint" />

  <joint name="base_joint" type="fixed">
    <parent link="footprint"/>
    <child link="chassis"/>
  </joint>

  <link name="chassis">
    <collision>
      <origin xyz="0 0 ${wheelRadius}" rpy="0 0 0" />
      <geometry>
        <box size="${chassisLength} ${chassisWidth} ${chassisHeight}" />
      </geometry>
    </collision>
    <visual>
      <origin xyz="0 0 ${wheelRadius}" rpy="0 0 0"/>
      <geometry>
        <box size="${chassisLength} ${chassisWidth} ${chassisHeight}" />
      </geometry>
      <material name="orange"/>
    </visual>
    <inertial>
      <origin xyz="0 0 ${wheelRadius}" rpy="0 0 0"/>
      <mass value="${chassisMass}"/>
      <box_inertia m="${chassisMass}" x="${chassisLength}" y="${chassisWidth}" z="${chassisHeight}"/>
    </inertial>
  </link>

</robot>

MACROS.XACRO

<?xml version="1.0"?>
<robot name="mybot_macros" xmlns:xacro="http://www.ros.org/wiki/xacro">
        <!-- Put here the robot description -->

  <xacro:macro name="cylinder_inertia" params="m r h">
    <inertia ixx="${m * (3 * r * r + h * h) / 12}" ixy="0" ixz="0"
      iyy="${m * (3 * r * r + h * h) / 12}" iyz="0"
      izz="${m * r * r / 2}"
    />
  </xacro:macro>

  <xacro:macro name="box_inertia" params="m x y z">
    <inertia ixx="${m * (y * y + z * z) / 12}" ixy="0" ixz="0"
      iyy="${m * (x * x + z * z) / 12}" iyz="0"
      izz="${m * (x * x + z * z) / 12}"
    />
  </xacro:macro>

  <xacro:macro name="sphere_inertia" params="m r">
    <inertia ixx="${2 * m * r * r / 5}" ixy="0" ixz="0"
      iyy="${2 * m * r * r / 5}" iyz="0"
      izz="${2 * m * r * r / 5}"
    />
  </xacro:macro>
</robot>

MYBOT.GAZEBO

<?xml version="1.0"?>
<robot name="mybot" xmlns:xacro="http://www.ros.org/wiki/xacro">
        <!-- Put here the robot description -->

  <gazebo reference="chassis">
    <material>Gazebo/Orange</material>
  </gazebo>
</robot>