Why can't the sub-component VUE be displayed?

Why can"t you display the contents of this Test.vue when you visit http://localhost:8080/?

App.vue Code

<template>
  <div id="app">
    <Test></Test>
  </div>
</template>

<script>
import Test from "@/components/Test"

export default {
}
</script>

<style>
-sharpapp {
  font-family: "Avenir", Helvetica, Arial, sans-serif;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  text-align: center;
  color: -sharp2c3e50;
  margin-top: 60px;
}
</style>

Test.vue Code

<template>
 <div>
  <button class="btn">{{msg}}</button>
 </div>
</template>

<script>
export default {
  name: "Test",
  data () {
    return {
      msg: "test"
    }
  }
}
</script>

<style scoped>
 .btn{background-color:blue;}
</style>
Apr.07,2021

in App.vue

export default {
    components: { Test }
}
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