Problem creating a slider in p5.js "Uncaught reference : createSlider is not defined"

Question

I am currently facing a problem in p5.js on openprocessing !

I try to add sliders to one of my sketches but the editor shows me an "Uncaught ReferenceError : createSlider is not defined"

Here is a part of my code

let slider;
let ellipses = [];
let colR = 10;
let colG = 10;
let colB = 10;


function setup() {
  slider = createSlider(0, 255, 0, 1);
  slider.position = (10, 10);
  slider.style = ('width', '255px');
  colR = random(255);
  colG = random(255);
  colB = random(255);
  createCanvas(windowWidth, windowHeight);
  background(100);

  noStroke()
}

function draw() {
  x = mouseX;
  y = mouseY;
  let val = slider.value();
  background(val);
  if (mouseIsPressed) {
    if (mouseButton == LEFT) {
      ellipses.push(new Ellipse());
    }
  }
  for (let i = 0; i < ellipses.length; i++) {
    ellipses[i].display();
    ellipses[i].move();
  }
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/p5.js/1.4.0/p5.min.js"></script>

the class Ellipse is a class I made that works just fine, and I don't think it is related to my problem.

Wondering why it wouldn't recognize the createSlider function, I tried creating a new sketch from scratch and simply make it a slider doing the same thing and just made this :

let slider;

function setup() {
    createCanvas(windowWidth, windowHeight);
    background(100);
    slider = createSlider(0,255,0,25);
    slider.position(10,10);
    slider.style('width','250px');
}

function draw() {
    let val = slider.value();
    background(val);
    
}

What surprised me here is that this works perfectly fine without anything else, on the same editor and website. So I went back to my original code but I just can't see what to do here to make it work as the code itself is absolutely the same. I also tried moving lines relating to the slider here and there but it doesn't change anything, I'm stuck.. Thx if anyone finds a solution to my problem :)