Why is there a "no suitable user-defined conversion type" error?

Question

Programming

The error "no valid user-defined conversion type exists".
I can't find the reason. I am using visual studio 2015.

struct point_str {
    double x_a;
    double y_a;
    double x_b;
    double y_b;
    double x_c;
    double y_c;
  };

  struct out_str {
    double radius_1;
    double radius_2;
    double interval;
  };

  int xmax_up;
  int ymax_up;

  vector<out_str> vostr;
  vector<point_str> vpstr_1;
  vector<point_str> vpstr_2;

  Class obj(image_source, image_curves_up, image_curves_down, xmax_up, ymax_up);
  obj.split_image(image_source, image_curves_up, image_curves_down);
  obj.calc_point_str(image_curves_up, image_curves_down, vpstr_1, vpstr_2);
  obj.calc_out_str(vpstr_1, vpstr_2, vostr, image_curves_up, image_curves_down, xmax_up, ymax_up);

Highlights in red in functions - vpstr_1, vpstr_2, vostr.