using Session dosen't work like in the diskription form Symfony

I tried this in my program sessions Basic Usage. I do it like this:

framwork.yml

    session:
    handler_id: 'session.handler.native_file'
    save_path: '%kernel.project_dir%/var/sessions/%kernel.environment%'
    cookie_secure: auto
    cookie_samesite: lax
    storage_factory_id: session.storage.factory.native

and I use a service to set and get the data from the Session.

class SessionService
 {   
  private $requestStack;
  public const  PRODUCT = 'product';
  public const AMOUNT = 'amount';
  private const SHOPPIN_CART= 'shopping_cart_products';

public function __construct(RequestStack $requestStack)
{
    $this->requestStack = $requestStack;   
}

public function setShoppingCart( Product $product, int $amount)
{
    $shoppingCart = $this->getShoppingCart();
    if ($shoppingCart == null){
        $shoppingCart[0][self::PRODUCT] = $product;
        $shoppingCart[0][self::AMOUNT] = $amount;
    }else{
        $id = count($shoppingCart)-1;
        $shoppingCart[$id][self::PRODUCT] = $prod;
        $shoppingCart[$id][self::AMOUNT] = $amount;
    }
    
    $session = $this->requestStack->getSession();
    
    $session->set(self::SHOPPIN_CART, $shoppingCart);
}

public function getShoppingCart():array
{
    $session = $this->requestStack->getSession();
    $shoppingCart = $session->get(self::SHOPPIN_CART);
    if($shoppingCart == null){
        return [];
    }
    return $shoppingCart;
}}

the problem is that the data arn't saved in the session or getting lost after the reload of the Side and I don't know why. Thanks for your help.