RikuXan,
The only way that error pops up is when the cells you are adding integer constraints to are not in the "By Changing Variable Cells" box. If you have different variable cells or nothing in that box, that error will pop up when you apply integer restraints.
For your example, you must have both the make-to-stock cell and the make-to-order cell in the "by changing variable..." box.
This is an extremely rare error message that I believe only students like us have trouble with, as I have found nothing else about this on the internet. I solved it for an exercise I was attempting shortly after I came across this post.
Hope that helps (I tried to post a picture but this is my first post). David