Invalid roslaunch XML syntax: not well-formed (invalid token):

edit

So, when I use:

<param name="robot_description" command="$(find xacro)/xacro.py $(find mastering_ros_robot_description_pkg)/urdf/seven_dof_arm.xacro" />

Appear this erro:

RLException: Invalid <param> tag: Cannot load command parameter [robot_description]: command [/home/jose/catkin_ws/src/xacro/xacro.py /home/jose/catkin_ws/src/mastering_ros_robot_description_pkg/urdf/seven_dof_arm.xacro] returned with code [2]. 

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

And when I use:

<command="$(find xacro)/xacro.py $(find mastering_ros_robot_description_pkg)/urdf/seven_dof_arm.xacro" param name="robot_description" />

Appear this:

RLException: Invalid roslaunch XML syntax: not well-formed (invalid token): line 3, column 9.
The traceback for the exception was written to the log file

@Jose-Araujo The format @jayess suggested is definitely right. You are setting a parameter called robot_description to a rosparam server by processing the seven_dof_arm.xacro file. This file needs to have this xacro header

<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="<your_robot>">

and needs to result in valid URDF file. That is if you use

rosrun xacro xacro path/to/seven_dof_arm.xacro > model.urdf

to convert the xacro file into URDF, and then

check_urdf model.urdf

to check validity of the URDF file.

It needs to say all is well formatted and print the structure tree into the terminal. So try those commands and see what it tells us.

It's running now. I find an error on xacro and changed what @kump and @jayess said. Thank you!

@Jose-Araujo That's great. Could you please click on the check mark (✓) to the left of the question to mark it as correct?