Display WordPress Sub-category in dropdown based on the selected Parent Category

I’m trying to show the parent categories in a drop-down box and based on which parent categories are selected, their Sub Categories should appear in a separate drop-down box via an ajax call.

I’ve done a lot of research and came across the following article which is 100% what I need to achieve: https://1stwebdesigner.com/implement-ajax-wordpress-themes/

I followed it correctly but for some reason, the sub-category drop-down is always disabled.

My end goal is to have the 2 dropdowns with a submit button and have it act as a search form, display the selected sub-category posts on a separate page, this was my first step to getting there but having no luck.

I’ve tried changing the javascript $ to jQuery as this was giving an error the first time I used it but currently no errors on the page at all.

<!-- Dropdown select box & javascript -->
<div id="content">
<?php
wp_dropdown_categories('show_count=0&selected=-1&hierarchical=1&depth=1&hide_empty=0&exclude=1&show_option_none=Main Categories&name=main_cat');
?>
<select name="sub_cat" id="sub_cat" enabled="disabled"></select>

<script type="text/javascript">
    jQuery(function() {
      jQuery('#main_cat').change(function() {
        var $mainCat = jQuery('#main_cat').val();

        // call ajax
        jQuery("#sub_cat").empty();
        jQuery.ajax({
          url: "/wp-admin/admin-ajax.php",
          type: 'POST',
          data: 'action=my_special_action&main_catid=' + $mainCat,

          success: function(results) {
            //  alert(results);
            jQuery("#sub_cat").removeAttr("disabled");
            jQuery("#sub_cat").append(results);
          }
        });
      });
    });
</script>

</div>

<!-- Ajax call in function.php -->

add_action('wp_ajax_my_special_action', 'my_action_callback');
add_action('wp_ajax_nopriv_my_special_action', 'my_action_callback');


function implement_ajax() {
if(isset($_POST['main_catid']))
            {
            $categories=  get_categories('child_of='.$_POST['main_catid'].'&hide_empty=0');
              foreach ($categories as $cat) {
                $option .= '<option value="'.$cat->term_id.'">';
                $option .= $cat->cat_name;
                $option .= ' ('.$cat->category_count.')';
                $option .= '</option>';
              }

              echo '<option value="-1" selected="selected">Scegli...</option>'.$option;
            die();
            } // end if
}
add_action('wp_ajax_my_special_ajax_call', 'implement_ajax');
add_action('wp_ajax_nopriv_my_special_ajax_call', 'implement_ajax');//for users that are not logged in.