Problem with Turtlebot pkg [closed]

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Hi Ros Users:

I have a problem with turtlebot pkg. I have just instaled and when I run

roslaunch turtlebot_gazebo turtlebot_world.launch

I can see this problem on the terminal:

> while processing
> /home/alcor/catkin_ws/src/turtlebot_simulator-indigo/turtlebot_gazebo/launch/includes/kobuki.launch.xml:
> Invalid <param> tag: Cannot load
> command parameter [robot_description]:
> command
> [/opt/ros/indigo/share/xacro/xacro.py
> '/home/alcor/catkin_ws/src/turtlebot-indigo/turtlebot_description/robots/kobuki_hexagons_asus_xtion_pro.urdf.xacro']
> returned with code [1]. 
> 
> Param xml is <param command="$(arg
> urdf_file)" name="robot_description"/>
> The traceback for the exception was
> written to the log file

How can I fix it?

Thanks!