想使用vue的router来跳转页面，但是报错识别不了push，如何解决？

想使用vue的router来跳转页面，但是报错识别不了push，在上面可以正常使用.$message弹出消息提示框

img

img

img

img

需要挂载在Vue实例中

<!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">
    <button @click="toAbout">toAbout</button>
    <router-view></router-view>
  </div>
  <script src="https://cdn.staticfile.org/vue/2.6.9/vue.min.js"></script>
  <script src="https://cdn.staticfile.org/vue-router/3.4.3/vue-router.min.js"></script>
  <script>
    const com = {
      template: `
        <div>
          <h1>about</h1>
        </div>
      `
    }
    const routes = [
      { path: '/about', component: com }
    ]
    const router = new VueRouter({
      routes
    })
    const App = new Vue({
      el: '#app',
      router,
      methods: {
        toAbout() {
          this.$router.push('/about')
        }
      }
    })
  </script>
</body>
</html>

向vue上 挂载router 你没挂载肯定this用不了