I am currently studying rrt alogrithm and I am having trouble with using the 2D nav goal feature. When I launch the unit4_demo.launch , I get this kind of errors which is shown in the captured image and nothing happens in the rviz when I try to use the 2D nav goal feature.
Did you start the mapping launch? Amcl? Could you state with more details the issue? Share some code and so on so we have a perfect idea of your question?