datepicker is not a function

Hi I get this error in the browser’s console:

Uncaught TypeError: jQuery(…).datepicker is not a function

/wp-content/plugins/users-ultra/js/uultra-front.js?ver=4.4

for the code:

// Adding jQuery Datepicker
jQuery(function() {
	jQuery( ".xoouserultra-datepicker" ).datepicker({changeMonth:true,changeYear:true,yearRange:"1940:2014"});

	jQuery("#ui-datepicker-div").wrap('<div class="ui-datepicker-wrapper" />');
});

https://ww.wp.xz.cn/plugins/users-ultra/