ImportError: No module named rospkg

edit

I am using ROS kinetic on Ubuntu 16.04. When I typed this command, "roslaunch turtlebot_gazebo turtlebot_world.launch", the following error occurred. Please teach me how to solve it....

 ... logging to /home/yuki/.ros/log/fafca31c-4c63-11e7-a590-ac2b6eec753e/roslaunch-yuki-Inspiron-13-7368-13183.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.

Traceback (most recent call last):
  File "/opt/ros/kinetic/share/xacro/xacro.py", line 55, in <module>
    import xacro
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/xacro/__init__.py", line 42, in <module>
    from roslaunch import substitution_args
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/roslaunch/__init__.py", line 48, in <module>
    import rospkg
ImportError: No module named rospkg
while processing /opt/ros/kinetic/share/turtlebot_gazebo/launch/includes/kobuki.launch.xml:
Invalid <param> tag: Cannot load command parameter [robot_description]: command [/opt/ros/kinetic/share/xacro/xacro.py '/opt/ros/kinetic/share/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