"She came to Beijing last fall."
The Chinese sentence is wrong, I reported already.
來到 came to (past only, it couldn't be now or future)
來過 have come to
她去年秋天來到北京 is proper answer.
她去年秋天來北京 is okay, but it seems the sentence is not over.
Or in Taiwan we say 她去年秋天來北京的
Both noun phrases are time (去年 and 秋天), juxtaposing them without a connector just makes a more specific time phrase. Same applies to e.g. location noun phrases. Adding 的 in these places sounds a bit clunky.
It's actually optional in other situations too, e.g. 我妈妈 is a common alternative to 我的妈妈 particularly in concise writing (when speaking, in chats, in newspapers)
Shouldn't "last fall" = "上个秋天" and "去年秋天" = "in fall last year"? (Of course for most of the year the two are the same, but in other places Duolingo is really nit-picky about similar distinctions.)