Hello.. please help..me..

ubuntu@ubuntu:~$ source /home/ubuntu/catkin_ws/devel/setup.bash
ubuntu@ubuntu:~$ roslaunch rplidar_ros view_rplidar.launch
... logging to /home/ubuntu/.ros/log/89ccce0a-e323-11e6-87cc-0022cffa2d4b/roslaunch-ubuntu-4625.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.

started roslaunch server http://ubuntu:54659/

SUMMARY
========

PARAMETERS
 * /rosdistro: indigo
 * /rosversion: 1.11.20
 * /rplidarNode/angle_compensate: True
 * /rplidarNode/frame_id: laser
 * /rplidarNode/inverted: False
 * /rplidarNode/serial_baudrate: 115200
 * /rplidarNode/serial_port: /dev/ttyUSB0

NODES
  /
    rplidarNode (rplidar_ros/rplidarNode)
    rviz (rviz/rviz)

ROS_MASTER_URI=http://localhost:11311

core service [/rosout] found
process[rplidarNode-1]: started with pid [4643]
process[rviz-2]: started with pid [4644]
RPLIDAR running on ROS package rplidar_ros
SDK Version: 1.5.7
Error, cannot bind to the specified serial port /dev/ttyUSB0.
[rplidarNode-1] process has died [pid 4643, exit code 255, cmd /home/ubuntu/catkin_ws/devel/lib/rplidar_ros/rplidarNode __name:=rplidarNode __log:=/home/ubuntu/.ros/log/89ccce0a-e323-11e6-87cc-0022cffa2d4b/rplidarNode-1.log].
log file: /home/ubuntu/.ros/log/89ccce0a-e323-11e6-87cc-0022cffa2d4b/rplidarNode-1*.log
[rviz-2] process has died [pid 4644, exit code -11, cmd /opt/ros/indigo/lib/rviz/rviz -d /home/ubuntu/catkin_ws/src/rplidar_ros/rviz/rplidar.rviz __name:=rviz __log:=/home/ubuntu/.ros/log/89ccce0a-e323-11e6-87cc-0022cffa2d4b/rviz-2.log].
log file: /home/ubuntu/.ros/log/89ccce0a-e323-11e6-87cc-0022cffa2d4b/rviz-2*.log
all processes on machine have died, roslaunch will exit
shutting down processing monitor...
... shutting down processing monitor complete
done