"The sheep goes over the zebra crossing."
Translation:Das Schaf geht über den Zebrastreifen.
As far as I'm aware, they generally do try to teach American English on Duolingo (which I wouldn't call International English, but maybe that's just me)—hence the American flag, rather than British, Canadian, Australian etc.—but they almost always accept alternatives from other variants.
I believe this sentence is an exception.
Let us be absolutely clear here: "the stripe" is "der Streifen" and "the stripes" is "die Streifen". However, "the zebra stripe" (crosswalk, zebra crossing) is a singular, masculine word " der Zebrastreifen", and the word for multiple crosswalks is "DIE Zebrastreifen". https://www.dict.cc/?s=Zebrastreifen
In duo's dictionary, when you insert the zebra crossing it gives you DIE Zebrastreifen
Duo's AI isn't perfect. If you type in multiple words, it has to use AI to decide the right form of a word in the right context, and it doesn't always get it right. (For what it's worth, "the zebra crossing" gave me "die Zebrastreifen," but "The zebra crossing" gave the correct "der Zebrastreifen.")
I recommend using a more reliable dictionary like Pons to find out the gender of a word.
"Über" is the correct word for passing over something. Depending on which case you put "Zebrastreifen" in, "auf" would mean that the sheep just walked onto the crossing (and just stopped there on it), or that it walked around on top of the crossing (without leaving it).