jQuery AutoComplete Error: Uncaught TypeError: $(...).autocomplete is not a function

autocomplete not working in master page

getting error: Uncaught TypeError: $(...).autocomplete is not a function

<script type="text/javascript">
    $(document).ready(function () {
        alert(  $("#<%=txtSearch.ClientID %>").val())
        $("#<%=txtSearch.ClientID %>").autocomplete({
            source: function (request, response) {
                $.ajax({
                    url: '<%=ResolveUrl("~/Admin/Service.asmx/GetCustomers") %>',
                    data: "{ 'prefix': '" + request.term + "'}",
                    dataType: "json",
                    type: "POST",
                    contentType: "application/json; charset=utf-8",
                    success: function (data) {
                        if (data.d.length > 0) {
                            response($.map(data.d, function (item) {
                                return {
                                    label: item.split('-')[0],
                                    val: item.split('-')[1]
                                };
                            }))
                        } else {
                            response([{ label: 'No results found.', val: 0 }]);
                        }
                    },
                    error: function (response) {
                        alert(response.responseText);
                    },
                    failure: function (response) {
                        alert(response.responseText);
                    }
                });
            },
            select: function (e, i) {              
                $("#<%=hfCustomerId.ClientID %>").val(i.item.val);               
            },
            minLength: 1
        });
    });
</script>