Thank you for your detail explanation. I tried the prohibition approach, but it doesn't work, even I have set the prohibition, still, some unexpected combinations showed when I tested it. Is this situation can be solved?
Related to the second approach, I read about premium pricing in chapter 3. As the example shown in the book, they were talking the situation only with two alternatives(which is small and large package price). However, in my case, I have four levels in each attribute (attribute 1 payback amount:£200,£300,£400, £500; attribute 1 repaying time : 1 month, 3 months, 4 months, 5 months). In this case, what should I do?