My mistake, in which case I couldn't tell you, although I know the English sentence is accurate. "I like fall but I don't like rains" would be wrong. It may be one of those cases where Russian simply works differently than English. I think rain is generally a mass noun in English, though I'm kind of fuzzy brained today so I wouldn't want to swear to that.

Rain is countable in English when it means a discrete rain shower or storm.

It was a long, heavy rain

The rains came early this year

The rains have already begun

If I remember right I was given the Russian and I typed English "rains" and it was accepted, so I interpreted it as "rain events in fall". But I don't remember. These threads are for the content that Duo Lingo Presents to you in several different ways. And if I were given the English "rain" and had to produce the Russian, I agree with you that using дождь would seem like the best answer to me. Because in both languages I would assume we were talking about rain in general, as an uncountable/collective/mass noun.

I see, the English sentence is gramatically right, just as you say — and the Russian one is gramatically correct too, the thing is they don't correspond each other since there is a variant with singular "дождь". Of course, the overall meaning of this Russian sentence doesn't change whether one uses "дождь" or "дожди", but still, I think, the main translation should be "Я люблю осень, но не люблю дождь."

Nobody was discussing here whether rain is countable or not, but thank you anyway.