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."
eni, thank you so much for your explanation. It's interesting that in Spanish we use an expression exactly like this in German: "No tiene sentido" a literal translation of Es hat keinen Sinn. I am discovering many German expressions exactly like we use in Spanish. Very interesting!
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 :)