Error while launching Rviz

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Hi,

I am new to Linux and ROS and just trying to set up MoveIt with the MoveIt Tutorials. I have followed the steps until section 2: MoveIt Quickstart in Rviz and now getting an error at:

roslaunch panda_moveit_config demo.launch rviz_tutorial:=true

This is the error I get:

... logging to /home/carlos/.ros/log/98d0fafc-a008-11ea-bd70-080027123d27/roslaunch-carlos-VirtualBox-2852.log
Checking log directory for disk usage. This may take a while.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

substitution args not supported:  No module named 'rospkg'
when processing file: /opt/ros/melodic/share/franka_description/robots/panda_arm_hand.urdf.xacro
RLException: while processing /home/carlos/ws_moveit/src/panda_moveit_config/launch/planning_context.launch:
Invalid <param> tag: Cannot load command parameter [robot_description]: command [['/opt/ros/melodic/lib/xacro/xacro', '/opt/ros/melodic/share/franka_description/robots/panda_arm_hand.urdf.xacro']] returned with code [2]. 

Param xml is <param command="$(find xacro)/xacro '$(find franka_description)/robots/panda_arm_hand.urdf.xacro'" if="$(eval arg('load_robot_description') and arg('load_gripper'))" name="$(arg robot_description)"/>
The traceback for the exception was written to the log file

Does somebody know how to fix the problem? System: Virtual Box Ubuntu 18.04, ROS Melodic and Python 3

Thanks in advance,

Carlos