Hi Momtaz,
To enable Gazebo Co-Plugin support, you need to add plugin details in .world file as,
For office world example, the turtlebot3_mw_office.world is located at /opt/ros/oetic/share/turtulebot3_gazebo/worlds/turtlebot3_mw_office.world
You need to edit turtlebot3_mw_office.world as follows,
Further, you should not use 11311 port number for connection. It should same which you added in .world file, with <portNumber> tag i.e. 14581
Thanks