Is it guaranteed that there is a foil rare in every booster box? I've recently finished opening a box and there was no foil rare and many duplicates. I already searched the web to see if it is guaranteed and couldn't find anything. I'm wondering if it was a printing/manufacturing error, or just bad luck.
No. It is a strong statistical possibility that a foil rare can be found in a box, but it isn't assured. It is statistically possible to open a booster box and find zero premium cards, as well as zero mythic rares.
The probabilistic expectation, if you were to open an innumerable amount of booster boxes, is that there will be six premium cards per booster box, with one of those six premium cards being a rare. Further, it is expected that one foil mythic rare will be opened for every eight booster boxes.