I want to add Purchase orders with my software, using the Odoo API. Adding the purchase order works fine, but adding lines throws an exception. 
-I have made sure all variables with "required" tags are included.
-If I add an order through the web page, and try to add lines to that one (with the API); results in the same error.

It's written in c#, Odoo version is 13.0

Code:

int orderId = -1;
{
    var properties = new
    {
        company_id = 1,
        currency_id = 1,
        date_order = "2020-09-28 08:00:08",
        name = "P00012",
        partner_id = 7,
        picking_type_id = 1
    };
    orderId = (int)model.execute_kw(DB, userId, password, "purchase.order", "create", new object[] { properties });
                
    Console.WriteLine("New order: " + orderId);
}
{
    var properties = new
    {
        order_id = orderId,
        product_id = 7,
        //product_uom = 5757,
        name = "Tavla",
        product_uom = 1,//Units of measure, is pointer with id
        product_uom_qty = 1,
        price_unit = 1.5,
        price_total = 1.5,
        product_qty = 1.0,
    };
    //The project crashes here:
    var res = model.execute_kw(DB, userId, password, "purchase.order.line", "create", new object[] { properties });
                
    Console.WriteLine("New order line: " + res);
}

Error message: