Datepicker'ın çalışması için jQuery UI'nin de dahil edilmesi lazım. Dahil etmediyseniz :

<script src="https://cdnjs.cloudflare.com/ajax/libs/jqueryui/1.13.2/jquery-ui.min.js" integrity="sha512-57oZ/vW8ANMjR/KQ6Be9v/+/h6bq9/l3f0Oc7vn6qMqyhvPd1cvKBRWWpzu0QoneImqr2SkmO4MSqU+R pHom3Q==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>

<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/jqueryui/1.13.2/themes/base/jquery-ui.min.css" integrity="sha512-ELV+xyi8IhEApPS/pSj66+Jiw+sOT1Mqkzlh8ExXihe4zfqbWkxPRi8wptXIO9g73F SlhmquFlUOuMSoXz5IRw==" crossorigin="anonymous" referrerpolicy="no-referrer" />