0๐
I made the following corrections and it started to work:
index.html
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8"/>
</head>
<body>
<div id="app" class="container-fluid">
<div class="row">
<div class="col-10">
<p><input type="button" value="Test DB" v-on:click="testDB"> <span v-html="message_db"></span></p>
</div>
</div>
</div>
<script src="js/vue.min.js"></script>
<script src="js/main.js"></script>
</body>
</html>
I updated script src tags. The order of script tags is important.
Vue.min.js refers to v2.3.0. I think https://unpkg.com/vue returns the last vesion de vue.js
About main.js file keeps the same content.
Source:stackexchange.com