the -ar and -arna forms for both sound differently to my ear, though.
"fall", though mostly used in the US today, actually comes from England and was once very common there as well. But either way, the course is aimed specifically at US English, with other major variations such as UK English also being accepted. Hence, "fall" is the default and "autumn" is also accepted. Please refer to the many other comments on this, as well as the FAQ: https://forum.duolingo.com/comment/20444477/FAQ-for-the-Swedish-course