i insert the record using simple php but record does not insert

hello everyone, i am using php i try to insert the record but record does not insert please anyone tell me what is issue? this is the code

 public function create() {
    $sql = "INSERT INTO bicycles (";
    $sql .= "brand, model, year, category, color, gender, price, weight_kg, condition_id, description";
    $sql .= ") VALUES (";
    $sql .= "'" . $this->brand . "', ";
    $sql .= "'" . $this->model . "', ";
    $sql .= "'" . $this->year . "', ";
    $sql .= "'" . $this->category . "', ";
    $sql .= "'" . $this->color . "', ";
    $sql .= "'" . $this->gender . "', ";
    $sql .= "'" . $this->price . "', ";
    $sql .= "'" . $this->weight_kg . "', ";
    $sql .= "'" . $this->condition_id . "', ";
    $sql .= "'" . $this->description . "'";
    $sql .= ")";
    $result = self::$database->query($sql);
    if($result) {
      $this->id = self::$database->insert_id;
    }
    return $result;
  }
 

this is my new.php file

<?php

require_once('../../../private/initialize.php');

if(is_post_request()) {

  // Create record using post parameters
  $args = [];
  $args['brand'] = $_POST['brand'] ?? NULL;
  $args['model'] = $_POST['model'] ?? NULL;
  $args['year'] = $_POST['year'] ?? NULL;
  $args['category'] = $_POST['category'] ?? NULL;
  $args['color'] = $_POST['color'] ?? NULL;
  $args['gender'] = $_POST['gender'] ?? NULL;
  $args['price'] = $_POST['price'] ?? NULL;
  $args['weight_kg'] = $_POST['weight_kg'] ?? NULL;
  $args['condition_id'] = $_POST['condition_id'] ?? NULL;
  $args['description'] = $_POST['description'] ?? NULL;

  // automatically send a construct method

  $bicycle = new Bicycle($args);
  $result= $bicycle->create();

  if($result === true) {

    $new_id = $bicycle->id;

    $_SESSION['message'] = 'The bicycle was created successfully.';
    redirect_to(url_for('/staff/bicycles/show.php?id=' . $new_id));
  } else {
    // show errors
  }

} else {
  // display the form

  $bicycle = [];
}

?>

<?php $page_title = 'Create Bicycle'; ?>
<?php include(SHARED_PATH . '/staff_header.php'); ?>

<div id="content">

  <a class="back-link" href="<?php echo url_for('/staff/bicycles/index.php'); ?>">&laquo; Back to List</a>

  <div class="bicycle new">
    <h1>Create Bicycle</h1>

    <?php // echo display_errors($errors); ?>

    <form action="<?php echo url_for('/staff/bicycles/new.php'); ?>" method="post">

      <?php include('form_fields.php'); ?>

      <div id="operations">
        <input type="submit" value="Create Bicycle" />
      </div>
    </form>

  </div>

</div>

<?php include(SHARED_PATH . '/staff_footer.php'); ?>