Failed to contact master at 172.x.x.x

edit

hi I'am trying to connect my pioneer LX Robot with my own system in which ROS Kinetic is installed for SLAM(in Ubuntu 16.04) via wifi,but after runnig this command

export ROS_MASTER_URI=http://172.x.x.x:11311

when i run

rosrun rosaria RosAria

every time i got the following error..

xyz@xyz-Desktop:~/catkin_ws$ export ROS_MASTER_URI=http://172.16.21.202:11311
xyz@xyz-Desktop:~/catkin_ws$ rosrun rosaria RosAria
[ERROR] [1494496662.614976996]: [registerPublisher] Failed to contact master at [172.16.21.202:11311].  Retrying...
Connnecting to robot using TCP connection to localhost:8101...
Could not connect to simulator, connecting to robot through serial port /dev/ttyUSB0.
ArSerialConnection::open: Could not open serial port '/dev/ttyUSB0' | ErrorFromOSNum: 2 ErrorFromOSString: No such file or directory
Could not connect, because open on the device connection failed.
Failed to connect to robot.

Any help is appreciated.