JavaScript

Guys Please help me , How do I link this HTML with Css and Js properly? 

window.onload = () => {
    startSetTimeoutAnimation();
  };
  
  function startSetTimeoutAnimation() {
    const refreshRate = 1000 / 60;
    const maxXPosition = 400;
    let rect = document.getElementById('rect0');
    let speedX = 0.01;
    let positionX = 25;
  
    window.setInterval(() => {
      positionX = positionX + speedX;
      if (positionX > maxXPosition || positionX < 25) {
        speedX = speedX * (-1);
      }
      rect.style.top = positionX + 'px';
    }, refreshRate);
    
  }

console.clear();
const navSlide= ()=> {
    const burger = document.querySelector('.burger');
    const nav = document.querySelector('.nav-links');
    const navLinks=document.querySelectorAll('.nav-links li');
 //Toggle Nav
    
    burger.addEventListener('click', () =>{
        nav.classList.toggle('nav-active');

    navLinks.forEach((link,index) =>{
       if(link.style.animation) {
           link.style.animation=''
        } else {

            link.style.animation=`navLinkFade 0.5s ease forwards ${index /7 + 0.2}s`;
        }
        console.log(index / 7);
    });
		burger.classList.toggle('toggle');
 });
}

navSlide();

Attempted to register a view with an id already in use: 2