Api retun http 404 when i try do add element to list?

In my application Api retun http 404 when i try do add element to list why? When i comment line in constuctor with adding element to list api return json with Order and work good.In my application Api retun http 404 when i try do add element to list why? When i comment line in constuctor with adding element to list api return json with Order and work good.

public class Order
{

    public List<pizza> Pizza { get; set; }


    public Order(int orderId, User user, string adress, string fullname, string zip_code, string city, string phonenumber, pizza pizza)
    {
        Pizza.Add(pizza);
        OrderId = orderId;
        OrderCreate = DateTime.UtcNow;
        user = this.user;
        SetZipCode(zip_code);
        AddressLine = adress;
        FullName = fullname;
        City = city;
        SetPhoneNumber(phonenumber);

  }

}


public class OrderService : IOrderService
{
    public readonly IMapper _mapper;

    public readonly IOrderRepository _orderRepository;

    public OrderService(IOrderRepository customerRepository, IMapper mapper)
    {
        _mapper = mapper;
        _orderRepository = customerRepository;
    }

    public Task CreateAsync(int orderId)
    {
        throw new NotImplementedException();
    }

    public async Task CreateAsync(int orderId, User user, string adress, string fullname, string zip_code, string city, string phonenumber, pizza pizza)
    {
            var order = await _orderRepository.GetAsync(orderId);
            if (order != null)
            {
                throw new Exception($"User with {order} already exist");
            }
            order = new Order(orderId, user, adress,fullname,zip_code,city,phonenumber,pizza);
            await _orderRepository.AddAsync(order);
    }

    public Task DeleteAsync(int orderId)
    {
        throw new NotImplementedException();
    }

    public async Task<Order> GetAsync(int OrderId)
    {

        var order = await _orderRepository.GetAsync(OrderId);
        return order;

    }

}
enter code here