It is required when you are calling someone's name instead of using their name in the sentence (in which case we would instead use 'o). If we were talking about Keoki, I might say, "He kāne 'o Keoki." ("Keoki is a man.") But when speaking to Keoki, instead I say, "He kāne 'oe, e Keoki." ("You are a man, Keoki.") Some people find it helpful to think of it as "hey": "Aloha e Keoki" = "Hey Keoki, Peace!"
Since in English we don't have anything like those markers, there really isn't a translation into English. Other languages have ways to mark those concepts, so you might say they have translations for those words, but not in English. I guess it has some sort of similarity to "the" and "a" in English in that you have to figure out when to use the right one and sometimes you don't use either. But if you've ever studied a language that has declensions and noun cases, then it would be pretty easy to grasp. "E" marks the vocative and "'o" marks the nominative for proper nouns. But if you don't know about noun cases, that probably won't help.
He = a, but a does not always equal he. E never means a, and ‘o never means the. In this context, you would be speaking directly to Keoki and calling his name, and therefore, before his name you need the word e.
The word ‘o is a totally different issue. You need it before the name when referencing the name as the subject of a sentence or when it is part of a verbless sentence starting with the name. ‘O Keoki kona inoa literally says Keoki his name or His name Keoki. The initial ‘O has no meaning in English, but it has to be there because you just cannot start a sentence with a name and nothing before it, ignoring the examples in the "polite expressions" section that go against this rule. That is yet another issue. Those are colloquial introduced speech exceptions from longer grammatically correct sentences, and they are exceptions that you will probably never hear or use.
We don't have one exact word for it in English. It means something like "peaceful brotherly love". You use it when meeting others as a way to wish them "peaceful brotherly love". You also use it when departing from someone to again wish them "peaceful brotherly love". It is a wonderful greeting to use, but doesn't really exactly translate as "hello", "greetings", or "goodbye". However, we often use it in exactly the same situations where in English we say, "Hello" or "Greetings", so when translating it into English, those are OK words to use. You will also sometimes see it translated as "love" or "peace" or other similar words.
By the way, you will sometimes hear it as, "Aloha kāua," which we could translate as, "Peaceful brotherly love between you and me." Or sometimes, "Aloha kākou," which is, "Peaceful brotherly love between all of us."
I suspect you had some other error you didn't notice. The software does not show you exactly what error you made. It just shows you the translation which is marked as the "Best Translation" regardless of what error you actually made. "Hello, Keoki" is a fine translation and I would be surprised if it wasn't already on the "accepted list". But just in case you didn't have another error and it really is a missing alternative, I hope you used the flag report to say "My answer should have been accepted."