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
Will germans understand if i use "wieder" ?
Das kann man wieder sagen. would be understood only literally, as "it's possible to say that now (but it wasn't in the past)".
Similarly with Das kannst du wieder sagen for the literal meaning of "you are able to say that again".
you can say that again which seems as if someone didn't hear and is asking to repeat, again?
This course is "German for speakers of English"; it's expected that people who take this course are reasonably fluent in English.
"You can say that again!" is a fixed expression in English which is "used to show that you completely agree with something that someone has just said" ( https://dictionary.cambridge.org/us/dictionary/learner-english/say_1 ) and which means more or less the same thing as "I agree with you completely" ( https://www.oxfordlearnersdictionaries.com/definition/english/say_1?q=say ).
I don't think that a native English speaker would interpret "You can say that again!" as "Please repeat what you just said".