Error in subscribing in local_planner_node.cpp

I added a new subscriber in the local planner node and then I did “rosnode info /local_planner_node” to check that the node is subscibing to the topic I need and happily it subscribes .however I get this error with the callback function
“error: invalid use of non-static member function” . Consequently I’m not apple to get the message I need from the topic