"Good afternoon, László!"
Translation:Jó napot, László!
Then the exercise should be translate "good day", not "good afternoon". It's not a common phrase in English anyway, but very antiquated. There's not some kind of linguistic void that is left by the exclusion of good afternoon. English speakers aren't going to toss in turn in their beds at night because they don't know how to say "good afternoon". It's like suggesting the Hungarian for "jerked chicken" is really "csirkes paprikas" because no one eats jerked chicken in Hungary: jerked chicken is not so fundamental to our understanding of the language that it is necessary to misconstrue its meaning.
"Good day" is simply just not used in everyday life, "good afternoon" is. (Note: I wouldn't see it as a problem the other way around.)
"Good afternoon" is used after "good morning" but before "good evening", just like "jó napot" is used after "jó reggelt" but before "jó estét." I don't think many people would attack translating "jó reggelt" as "good morning" & definitely no one would have a problem with translating "jó estét" as "good evening." "Good afternoon" just fits.
This is how it's usually translated the other way around. At least, when I was learning English in my grammar school, "good afternoon" was the equivalent of "jó napot." If it's not broken, I don't think it needs to be fixed.
I'm not currently doing the Hungarian course, so I can't see the course notes right now. But what it needed here, is to include an explanation about Hungarian greetings if there isn't already one available. Saying when to use which, clarifying what reggel means &c. If there's no such thing, let the creators know, please, if there is, please (again) do read it.
Just like they have already said, when translating greetings, the point of the translation is to map actual greetings to actual greetings... "Jó délutánt" is a nonexistent greeting in Hungarian, just like "Good day" would be rare in English. You are expected to write something that you would actually say if you were to communicate using a different language, in an analoguous situation, and this is only right, at this point.
Fun fact If you think you're ready for more informal communication, here's how you can say thanks: First, drop the 'szépen'. That makes it sound very formal. Then, you basically chop the word in half, so the informal 'thanks' would be 'kösz', or more commonly 'köszi' because that sounds a bit nicer. You can also playfully say 'köszi szépen' but only in informal situations. You could also say 'kösz szépen' but I think, as a native, that it makes it sound a bit more sarcastic. Maybe it's just me. For more emphasis you say 'nagyon köszi' which means 'thanks a lot' or 'thanks very much'. I promise it sounds less sarcastic in Hungarian.