Opencv error when trying to run ORBSLAM2

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I installed ROS Kinetic and opencv3.3.1 on my laptop with the Ubuntu 16.04 LTS. I followed the instructions from the https://github.com/raulmur/ORB_SLAM2 and tried to use the embed camera from the laptop to run the ORBSLAM2. It pop up errors when using the command: rosrun ORB_SLAM2 Mono /home/babystella/catkin_ws/src/ORB_SLAM2/Vocabulary/ORBvoc.txt /home/babystella/catkin_ws/src/ORB_SLAM2/Examples/ROS/ORB_SLAM2/ost.yaml

Errors showing below:

Input sensor was set to: Monocular
OpenCV Error: Unknown error code -49 (Input file is empty) in cvOpenFileStorage, file /tmp/binarydeb/ros-kinetic-opencv3-3.3.1/modules/core/src/persistence.cpp, line 4484

terminate called after throwing an instance of 'cv::Exception'
  what():  /tmp/binarydeb/ros-kinetic-opencv3-3.3.1/modules/core/src/persistence.cpp:4484: error: (-49) Input file is empty in function cvOpenFileStorage
Aborted (core dumped)

I have searched the Internet, and had no idea about the problem. It will be great help if anyone could help me.