Gentelella Combo Box

Question

I am using combo box - select custom, using gentelella template. On example , gentelella select custom, we can type text on the combo box and the list on the combo box will change accordingly to the text we type.

But In my case, I can not type the text on the combo box. It won't work.

this is my view

<div class="item form-group">
                                    <label class="control-label col-md-3 col-sm-3 col-xs-12">NIK</label>
                                    <div class="col-md-6 col-sm-6 col-xs-12">
                                        <select class="select2_single form-control" tabindex="-1">
                                            <option></option>
                                            <?php foreach ($listkary as $key => $value) { ?>
                                                <option value="<? echo $value['idnik']?>"><? echo $value['kodenik']?> - <? echo $value['namakary']?></option>
                                            <? } ?>
                                        </select>
                                    </div>
                                </div>