carla::rpc::Command::SetAutopilot() didn't work

[Yandong-Luo]

CARLA version: 0.9.14
Platform/OS: Ubuntu 20.04
Problem you have experienced:
I am trying to build a traffic scenario based on C++ API. I have spawned all the npc vehicle, and tried to set the autopilot mode. But npc vehicle didn't move. I think the carla::rpc::Command::SetAutopilot() function didn't work.
My code is here:

int npc_number = 10;
    auto map = carla_world.GetMap();
    auto blueprint_library = carla_world.GetBlueprintLibrary();
    auto all_vehicles_bl = blueprint_library->Filter("vehicle");
    auto recSpawnPoint = map->GetRecommendedSpawnPoints();
    std::vector<carla::rpc::Command> batch_command;
    // auto npc_command = new carla::rpc::Command::SpawnActor();
    for(int i = 0; i < npc_number; ++i){
      auto npc_blueprint = RandomChoice(*all_vehicles_bl, rng);
      // Randomize the npc_blueprint color.
      if (npc_blueprint.ContainsAttribute("color")) {
        auto &attribute = npc_blueprint.GetAttribute("color");
        npc_blueprint.SetAttribute(
            "color",
            RandomChoice(attribute.GetRecommendedValues(), rng));
      }
      // Randomize the npc id.
      if (npc_blueprint.ContainsAttribute("driver_id")) {
        auto &attribute = npc_blueprint.GetAttribute("driver_id");
        npc_blueprint.SetAttribute(
            "driver_id",
            RandomChoice(attribute.GetRecommendedValues(), rng));
      }
      npc_blueprint.SetAttribute("role_name","autopilot");
      auto npc_transform = recSpawnPoint[i];
      // npc_description.
      auto npc_description = npc_blueprint.MakeActorDescription();
      carla::rpc::Command::SpawnActor npc_spawn(npc_description, npc_transform);
      npc_spawn.do_after.emplace_back(carla::rpc::Command::SetAutopilot(npc_description.uid, true, tm.Port()));
      // batch_command.emplace_back(auto_npc);
      batch_command.emplace_back(npc_spawn);
      // batch_command[i] = npc_spawn;
      std::cout<<"finish npc:"<<npc_description.uid <<"port"<<tm.Port()<<std::endl;
    }
    // excute the command for npc vehicle
    auto excute_responses = client.ApplyBatchSync(batch_command, true);
    std::vector<carla::rpc::ActorId> npc_id; 
    for(auto response : excute_responses){
      if(!response.HasError()){
        npc_id.emplace_back(response.Get());
      }
    }