vue代码无法运行，提示Unused constant app，如何解决？

代码及报错如下

img

没看到JS文件引入

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <div id="app">
    <h1>{{ msg }}</h1>
  </div>
  <script src="https://cdn.bootcdn.net/ajax/libs/vue/2.6.14/vue.min.js"></script>
  <script>
    const app = new Vue({
      el: '#app',
      data: {
        msg: 'Hello Vue'
      }
    })
  </script>
</body>
</html>

没有导包