Well, actually, in British English, we can say "strawberry" when we mean "anything done/cooked/stewed/baked containing strawberries", which makes it something uncountable opposed to countable berries.
That said, it is probably best to stick to "strawberries" when not sure ...
I recommend that you get a human teacher. They can more easily judge these things.
Duo is a computer. It can only match answers against a list of accepted sentences. If there is any difference, it will be rejected, because Duo doesn't actually understand Turkish or English.
Please read this comment from "leighozdemir": You've got the right idea, but in English we would say "strawberries". So, they can add another sentence in data bank. I guess all of us know Duo is a computer. But admins are not computers. By the way, thanks for your recommendation.
These pairs of sentences are also used in the English for Turkish speakers course, so it's best to go with a sentence that is (as far as I know) correct everywhere. Using "strawberry" would sound wrong in many parts of the English-speaking world. Using "strawberries," on the other hand, would be correct everywhere.
+in, +ın are very old adverb suffixes (zarf eki) in Turkish, however they're kind of dead in the current Turkish language and only some expressions formed using them survive. Some examples: Yazın (during summer), kışın (during winter), güzün (during fall), gündüzün (during day time), demin (just now), ilkin (at first), etc.
While Turkish is normally very reliable, there are a few irregular words. Although it is logical, "kışda" is incorrect, and that strange looking "kışın" is actually supposed to be there.
This same irregularity occurs with the Turkish word for "summer": "yaz" will become "yazın" (in summer). But spring and autumn are regular: ilkbaharda / sonbaharda.
Thankfully there aren't too many irregularities to worry about, but it can definitely be confusing when we find them. I hope that helps :-)
Also as a side note: "winter strawberry" would be a compound noun (suffix -i, not -in)... with consonant mutation on the "k" between two vowels. So we'd end up with "kış çileği".