ASP。Net MVC创建一个注册错误

I have this thing running at my application. I have 3 tables, 1 main table and 2 other tables that are needed to create the main table, the main table depends on the 2 tables, they are connected with constraints. I have the main table: Word Then I have the two other tables: Difficulties and Category. I created the Difficulty: Easy and then I created the Category: Nature. So now, I would be able to create as many words as I could with those 2 attributes, but it gives me an error. I can only create a word if I create this way I have this thing running at my application. I