Even when referring to a single cup, English usually favors the substantive "coffee" rather than "a coffee." Even when not "kahvia," the articleless "coffee" is a more natural translation. "A coffee" is usually reserved for times when a number of cups needs to be specified, such as when ordering at a restaurant, not when offering someone coffee. The above translation is appropriate, especially to being asked by a waiter or waitress who doesn't have a coffee pot in hand, but it really needs to accept the zero article version as well.
I was actually stuck on how to translate this into English with the words provided since I was translating it in my head as "Here's your coffee." I would definitely recommend that "coffee" and "a coffee" both be accepted at the very least, but "Here is your coffee" is probably the best even though I understand this may cause confusion since no possessive words are used.
Something I have noticed going through this training is that the amount of "interpretation" of the sentence varies. In some instances the sentiment of the sentence is reflected in the translation (here is your coffee) but at other times a more direct translation is required (coffee, be good). In this case I opted for the middle ground of "Coffee, here you are" as this might often be said at a cafe or if someone made a coffee for you. I don't think I have ever heard someone say "Here you are, a coffee". It is getting rather difficult and annoying to try and guess the correct level.
The translation as given looks weird to me. More like the speaker means "in this place (and not in another) you are a coffee (and not a person)." But then I am from places where we say "here you go" when we hand someone something, and while one might order "...and a coffee" with one's cinnamon roll, when handing it over they are more likely to say "your coffee "
Dedicated to the moderator: when do you actually take notice of what the people here is telling you. Coffee or a coffee. Here you are, there you are here's your coffee. Blah blah. Get it right pleased. You had plenty of contents here to take notice. By the way sisu does not exist in English.