Native speaker here. There is nothing wrong with "again" in this sentence. The most often used expression is "You can say that again", but given this is a translation exercise, replacing "you" with "one" is fine as well.
Why does "Das kann man wohl sagen." mean "You can say that again". I checked a German dictionary and it means "probably", "surely", "perhaps", "well" or "about" but I've never found that "wohl" means "again". Except in the case this sentence is idiomatic (then, it should be put in an idiomatic lesson), my sentence "You can say that probably" should had been accepted.
I think Duolingo does not handle the german passive "man" particle well. Whenever there is a "man" in the sentence, you can never know what it will accept. I have always been taught that "man" is a subject that reflects an anonymous actor in the sentence and that it can be replaced by a passive sentence without subjects in other languages. Am I wrong?
christian explains it well, and succinctly:
another correct answer is : You said it. = You can say that again. These are English idioms. https://idioms.thefreedictionary.com/you+can+say+that+again