error while trying to use rviz to visualize the urdf

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I am trying to use the rviz package to visualize the urdf I had made using a launch file

but every time I am trying to roslaunch my launch file give me this error

I have double checked every thing but the error still appear

 roslaunch mastering_ros_robot_description_pkg view_demo.launch
[view_demo.launch] is neither a launch file in package [mastering_ros_robot_description_pkg] nor is [mastering_ros_robot_description_pkg] a launch file name
The traceback for the exception was written to the log file