Js doesnt completely work in a .js file, but works in a html file with the <script> tag-CodePudding

I'm having trouble understanding why my volume slider doesn't work whenever the JS is in a JavaScript file, but works whenever its in a html file with the < script> tag.

<!DOCTYPE html>
<html>

  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title>wynn</title>
    <link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.15.4/css/all.css">
    <link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.15.4/css/v4-shims.css">
    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/[email protected]/dist/css/bootstrap.min.css">
    <link rel="stylesheet" href="style.css">
    <script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/jquery.slim.min.js"></script>
    <script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/umd/popper.min.js"></script>
    <script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/js/bootstrap.bundle.min.js"></script>
    <script src="https://twemoji.maxcdn.com/v/latest/twemoji.min.js" crossorigin="anonymous"></script>
    <script src="script.js"></script> <!-- this has the same thing as the bottom JS has. -->
    <script>
      window.onload = function() {
        twemoji.parse(document.body, {
          folder: 'svg',
          ext: '.svg'
        });

      }

    </script>
    <style>
      img.emoji {
        width: 20px;
        pointer-events: none;
      }

    </style>
  </head>

  <body>

    <div >
      <div  style="width: 500px;">
        <div >
          <!-- BANNER -->
          <img src=""  style="width: 400px; height: 0px;">
          <!-- PFP   NAME -->
          <h5 ><img src="https://cdn.discordapp.com/avatars/668093899683790859/a_57ab56ba8ed8be7ffef2686a81129ac3.gif"  style="width: 100px; height: 100px;">wynn</h5>
          <!-- DESCRIPTION -->
          <p >i am wynn u know me from some places or you're just clicking on random peoples profiles on discord, either way hi