This sentence is usually used when something just isn't worth trying anymore.
Let's say your car won't start and you try several things to get it starting again. Someone tells you to try again, and you answer annoyed: "Es hat keinen Sinn!". In that situation, you basically say: It is meaningless (to try again), or: There's no point/sense (in trying again). It's useless ("es ist sinnlos").
You also hear often "Es ergibt keinen Sinn!" (This doesn't make sense!) as in: I don't understand how this works, or why this won't work. Also often: "Es macht keinen Sinn!", which is an anglicism of that sentence, coming from "It makes no sense.", e.g. "The book makes no sense!" (Das Buch ergibt/macht keinen Sinn!).
"It has no reason", translates IMHO to "Es hat keinen Grund (etwas zu tun).": "Es (das Kaninchen) hat keinen Grund dich zu beissen." aka "It (the bunny) has no reason to bite you."
I used 'It makes no sense", which seems like it would be the closest English idiom?
No, it means "pointless, don't even try doing it, useless". Doesn't make sense - "nicht sinnvoll". Or as presented in this section "macht keinen Sinn", but I've heard at least three German friends say that they don't like/use it and here on Duo "local" Germans said the same:
All of that is true, but i'd clarify, that there are people who don't like it when others say "macht Sinn" instead of "hat Sinn", oder "ergibt Sinn", because they see it as an anglicism, which historically is correct, but if you like something or not is still an opinion.
Apart from that, refering to the former comment, i'd point out, that "purpose" would usually have another translation in German, which is "Zweck". Both are closely related, there is even a phrase sort of combining the two, "Sinn und Zweck...", but there are differences, as "Zweck" usually describes a practical aspect, some "purpose", while "Sinn" refers more to the meaning, as a construct, an ideology, a sense, and possibly some additional things. For a text for example, the words and sentences you're reading usually transport or may have a meaning for you, while the purpose of the document might be something entirely different.
However, the sentence "it is useless" could be translated with both in German, "es hat keinen Sinn", oder "es hat keinen Zweck", as the phrase usually comes up in the context of a situation which seems unsolvable, which can be both, due to an attempt not fulfilling its purpose, or something in it not having a sense or meaning.
Hope this helps more than it confuses :)
Technically you could also translate "it is useless" as "es ist nutzlos" which is yet another different meaning... The power of this language is its synonyms, roots and word creation...
With German, if it confuses, it helps! :D so thanks for both
Why not "it is meaningless"? Especially considering that "it is pointless" is accepted ...
It would mean pretty much the same, though the closer translation for "it is meaningless" would be "es ist sinnlos". But, well, if "it is pointless" is accepted, why shouldn't the former be.
A German told me that the word used here is "ergeben", i.e., "Es ergibt keinen Sinn" is that right?
That would mean "It doesn't make sense" as in, this is nonsense, "Es ist nicht sinnvol". And "hat keinen Sinn" means "It's useless" as in, pointless to try/resist/argue/...
I said "It has got no meaning!" which to me makes most sense in the context of spiritualism. Can't see how "It is useless!" would fit in with religion, though that would be the best translation and most common use.
Duo's point is probably that the English word sin is very different, and shouldn't be used the same way in German. It's a useful lesson. I mean, I'm glad I won't be making that error.