// Clear Stock
function clear_current_stock(){ 
// Declare the $wpdb var
    global $wpdb;
// Run My SQL Query (My Query is setting the meta value for stock to zero)
    $wpdb->query("UPDATE wp_postmeta SET meta_value = 0 WHERE meta_key = '_stock'");
// Check if there were any errors or if my query was successful.
    if ( is_wp_error( $wpdb ) ) {
         echo $wpdb->get_error_message();
    }
    else {
         echo 'true';
    }
}

Then you can call the function like this

<?php clear_current_stock(); ?>

<?php
  $user_id = get_current_user_id();
  if (!empty($user_id)) {
  $user = wp_get_current_user();
  $role = ( array ) $user->roles;
  // echo $role[0];
  if ($role[0] == 'free-client' OR $role[0] == 'administrator') {
    //code here...
  }
  }
?>

<?php
// set the expiration date to one hour ago
  setcookie("user", "", time() - 3600);
?>

<?php
  echo "Cookie 'user' is deleted.";
?>

<?php
  $cookie_name = "user";
  $cookie_value = "Alex Porter";
  setcookie($cookie_name, $cookie_value, time() + (86400 * 30), "/");
?>

<?php
  if(!isset($_COOKIE[$cookie_name])) {
       echo "Cookie named '" . $cookie_name . "' is not set!";
  } else {
       echo "Cookie '" . $cookie_name . "' is set!<br>";
       echo "Value is: " . $_COOKIE[$cookie_name];
  }
?>

// Custom Post 1
    register_post_type( 'courses',
        array(
            'labels' => array(
                'name' => __( 'Courses' ),
                'singular_name' => __( 'Course' )
            ),
        'public' => true,
        'has_archive' => true,
        'supports' => array('title', 'editor', 'thumbnail'),
        'show_ui' => true
        )
    );
 register_taxonomy('courses_categories', 'courses', ['label' => 'Courses Categories','hierarchical' => true]);

NB! (The hierarchical array item is Important)
,’hierarchical’ => true