"Jean-Paul slipped on a banana peel."
Translation:Jean-Paul a glissé sur une peau de banane.
We use both - I definitely use "peel", probably more often than "skin". But it's perfectly understandable what it means and the point here is to learn what it is in French.
If you had the exercise where you translate it freely, i.e. not with word tiles, and they didn't accept "skin", you are free to flag that up (literally: use the flag and explain your problem at the bottom of the list), and they'll undoubtedly add it to the list of acceptable words. But as it's apparently a US-based company, you (and the rest of us non-US Americans) just have to accept that they're going to use their terms over anyone else's. Though that last sentence comes with the caveat that many an American has complained about their English, too!
If they're just plain wrong (which does happen), we all tell them!