скачал 6.0.0 и распаковал. Мда, что-то не так с путями.
<html>
<head>
<title>Hello World</title>
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no">
<link rel="stylesheet" type="text/css" href="ext-6.0.0/build/classic/theme-neptune/resources/theme-neptune-all.css">
<script type="text/javascript" src="ext-6.0.0/build/ext-all-debug.js"></script>
<script type="text/javascript" src="ext-6.0.0/build/classic/theme-neptune/theme-neptune.js"></script>
<script>
Ext.onReady( function() {
Ext.create("Ext.panel.Panel", {
title: "Panel",
renderTo: Ext.getBody()
});
});
</script>
</head>
<body></body>
</html>