After class today some students came up to me and said they were having trouble with the homework, the part where there is transfer pricing but also an external market.
I just tried it and it is fine as is. But I also tried to replicate the error and I think I did that. The problem is that there is a price in cell F255. You see a number with two decimals, but you were instructed in the tutorial not to trust what you see but rather to use the cell reference. See if that works for you.
(I'm placing the same message as a comment here.)