RLException: Invalid <param> tag: Cannot load command parameter [robot_description]: command

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The error is that the <param> is in the top-level scope of the launch file. In ROS 2, <param> tags must be contained inside of <node> tags. Try changing your launch script to look more like this:

<?xml version="1.0"?>
<launch>

<node name="robot_state_publisher" pkg="robot_state_publisher" type="state_publisher">
  <param name="robot_description" command="$(find xacro)/xacro --inorder '$(find robufast_base)/launch/robufast.urdf'" />
</node>

<!--<node name="rviz" pkg="rviz" type="rviz"/>-->

<arg name="gui" default="true" />

<node name="joint_state_publisher" pkg="joint_state_publisher" type="joint_state_publisher" />

<!-- You'll need to a figure what node is consuming this parameter and pass it directly to the node -->
<!--param name="use_gui" value="$(arg gui)"/-->

</launch>

The robot_description parameter should be passed directly to robot_state_publisher. As my comment in the above code states, you'll also need to move the use_gui parameter inside of tags for the node that needs it.

The above launch script will only work with the latest version of robot_state_publisher (ROS 2 Foxy or later).This is because it was changed to read the robot description from a parameter (like in ROS 1) in https://github.com/ros/robot_state_pu....

For ROS 2 Eloquent, or older, you'll need to pass the robot description file as a command line argument. For example,

<node name="robot_state_publisher" pkg="robot_state_publisher" type="state_publisher"
    args="$(find robufast_base)/launch/robufast.urdf'" />
</node>

With the caveat that robufast.urdf above has already been expanded by xacro.