Based on your comment, you say you haven't included jQuery, jQuery UI or the jQuery UI CSS file.
All of these components are needed to make the $(".datepicker").datepicker();
function work. You can either upload and reference these as static resources or you can link to a CDN. The links to Google's jQuery CDN are below:
<!-- jQuery -->
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<!-- jQuery UI -->
<script src="https://ajax.googleapis.com/ajax/libs/jqueryui/1.12.1/jquery-ui.min.js"></script>
<!-- jQuery UI CSS -->
<link rel="stylesheet" href="https://ajax.googleapis.com/ajax/libs/jqueryui/1.12.1/themes/smoothness/jquery-ui.css" />