Uncaught ReferenceError: Vue is not defined when put vue setting in Index.html

i recently learning about vue

I have this file main.js

import Vue from 'vue/dist/vue.js'
import Buefy from 'buefy'
import 'buefy/lib/buefy.css'
Vue.use(Buefy)

var App = new Vue({
    el: '#app',
    data: {
          message : "It's working"
    }
})

and here is my html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <title>Vue Example</title>
  </head>
  <body>
    <h3 id="app">{{ message }}</h3>
    <script src="dist/build.js"></script>

    <script>

    </script>
  </body>
</html>

It's working. But, now i'm trying to do something with my script. I change main.js (I'm using webpack)

import Vue from 'vue/dist/vue.js'
import Buefy from 'buefy'
import 'buefy/lib/buefy.css'
Vue.use(Buefy)

then my index.html to this

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <title>Vue Example</title>
  </head>
  <body>
    <h3 id="app">{{ message }}</h3>
    <script src="dist/build.js"></script>

    <script>
var App = new Vue({
    el: '#app',
    data: {
        message:"It's not working"
    }
})
    </script>
  </body>
</html>

and i get this error

Uncaught ReferenceError: Vue is not defined

How can i fix this ?