Unable to update the EntitySet because it has a DefiningQuery and no InsertFunction element exists in the ModificationFunctionMapping element to support the current operation

You need to have Primary key in your table in which you are saving details, this error occurs as when EF mapping is done with a table which doesnot have Primary key, it is considered as a view & views being logical enity, cant be updated.

So adding PK in your table would work.

I was getting the same error as described in question, in the below code and solved it with the help of @pika's answer

               Student_Marks stuDentMarks = new Student_Marks();
                stuDentMarks.Student_Id = int.Parse(model.Student_Id);
                stuDentMarks.Marks = int.Parse(model.stuDentMarks);
                context.AddToStudent_Marks(stuDentMarks);
                context.SaveChanges();

Solution: I rechecked Table Student_marks in Database and Primary Key was not set in the table , I set the primary key and above problem was resolved