service error

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hi
this is my service client code

include "ros/ros.h"
include "scenario/scenario.h"
include <cstdlib>

int main(int argc, char **argv) {
ros::init(argc, argv, "remote");
ros::NodeHandle n;
ros::ServiceClient client = n.serviceClient<scenario::scenario>("stringa");
ros::Rate loop_rate(0.5);
while (ros::ok()){
scenario::scenario srv;
srv.request.pc="hello word";
if (client.call(srv)){
if((unsigned)srv.response.rasp[0]!=255)
ROS_INFO("%s", srv.response.rasp);
} else
{
ROS_ERROR("Failed to call service scenario");
return 1;
}
ros::spinOnce();
loop_rate.sleep();
}
return 0;
}

but when i launch the program receive ERROR Failed to call service scenario, what is the problem?
P.S i create the service that is called scenario