Sort by filter is not visible in Webpage in virtuemart

Question

I am new to Joomla community. In product category in virtuemart, I have setup sorting fields like sort the products by Alphabetically manner, new and old products.I have written php code in default.php. But dropdown is not visible over the webpage. Below is the code of PHP.

<span class="category-order-by" id="custom-category-order-by">
                    <div class="form-group">
                      <label for="custom_orderby"><?php echo vmText::_ ('COM_VIRTUEMART_ORDERBY') ?></label>
                      <select class="form-control select-picker" id="custom_orderby" name="custom_orderby" onchange="document.location.replace(this.value);">
                        <option disabled selected>Select an option</option> 
                        <option <?php if(strpos($url,'/by,product_sales?keyword=') !== false) {?> selected <?php } ?> value="<?php echo $urlPath ?>/by,product_sales?keyword=">Best Selling</option>
                      </select>
                    </div>
                </span>