It can be useful to reproduce the error message you get when asking for help / reporting an error.
If I paste your code, I get the message: "processing.app.SketchException: Found one too many { characters without a } to match it" and it highlights the opening brace of the constructor.
If you look at the { and the }, you see there are more { than }. The } of the class is missing.

the {} thing is maybe just because he forgot to copy one } from his Code to the forum in a correct way.

Next problem is that you don't have a Class Rectangle. There's none. Maybe write your own. But for now

maybe:

1. class Attractors
2. {
3.   //Rectangle
4.   PVector rectanglePos;
5.   PVector rectangleSize;
6.   // other 
7.   PVector v2;
8.   float thresh = 50;
9.   Attractors(int x, int y, int w, int h) {
10.     // a = new Rectangle(x, y, w, h);
11.     rectanglePos = new PVector (x, y, 0);
12.     rectangleSize = new PVector (w, h, 0);
13.     v2 = new PVector();
14.     v2.set(rectanglePos);
15.   }
16. }

or as a whole program:

BTW probably you want rather say Attractor in singular for the class and not use plural

1. Attractors n1 = new Attractors(10, 10, 20, 40);
2. void setup() {
3.   size(660, 660);
4. }
5. void draw () {
6.   n1.draw();
7. }
8. // ===========================================
9. class Attractors
10. {
11.   //Rectangle
12.   PVector rectanglePos; // use only x and y of it
    
13.   PVector rectangleSize;  // dito for size
    
14.   // other 
15.   PVector v2;
16.   float thresh = 50;
17.   //
18.   Attractors(int x, int y, int w, int h) {
19.     // a = new Rectangle(x, y, w, h);
20.     rectanglePos = new PVector (x, y, 0);
21.     rectangleSize = new PVector (w, h, 0);
22.     v2 = new PVector();
23.     v2.set(rectanglePos);
24.   }
25.   //
26.   void draw () {
27.     rect (rectanglePos.x, rectanglePos.y,
28.     rectangleSize.x, rectangleSize.y);
29.   }
30.   //
31. } // class
32. // ===========================================

Greetings, Chrisir