clang: error: linker command failed with exit code 1

I was doing the 12th chapter in Bjarne's 2nd edition of "Programming: Principles and Practices using C++" in which i have to install Fltk and alo use many other header files in the project. I successfully installed Fltk and build it but i don't know how to link the Fltk file and other necessary file in a single project. I have tried a lot of things but nothing is working. The code i wanna run is this:

#include "Simple_window.h"
#include "Graph.h"


int main()
{
  using namespace Graph_lib;
  
  Point tl {100,100};
  
  Simple_window win {tl,600,400,"Canvas"};
  
  
  Polygon poly;
  
  poly.add(Point{300,200});
  poly.add(Point{350,100});
  poly.add(Point{400,200});
  
  poly.set_color(Color::red);
  
  win.attach (poly);
  
  win.wait_for_button();
}

This is my project and the files i have tried to link and failed:

And this is what i am getting as the error, even after lot of searching on the internet, i can't figure out how to solve this issue:

pls tell me what i am doing wrong, i am very new to Xcode. Also if there is another way to make the program without Xcode, then that will help too. Even though i got the same error while making the program using Make.

clang: error: linker command failed with exit code 1
 
 
Q