Ο άντρας θέλει έναν καφέ should be accepted too. Since the first letter after the indefinite article is a plosive consonant (κάππα), the indefinite article takes up a νι.
I have noticed that Greeks themselves vary on this too.
Hey, this is also an accepted translation. Did you receive an error?
I got an error. The word έναν was underlined.
Actually, what you wrote should be the only right answer. While all Greeks would understand what you want if you said θέλω ένα καφέ, if we go by the rule θέλω ένα καφέ means I want a café/coffee house.
So, the correct answer is Θέλω έναν καφέ.
I used άνδρας and received an error.
άνδρας not accepted here ;)
Αλλα γιατι δεν χρησιμοποιω "Ο αντρασ θελει ενασ καφεσ" αυτο ειναι το ιδιο;
ένας καφές is the nominative (subject) form. Since the coffee is being wanted by the man, it is the object and should be in the accusative (object) form: ένα(ν) καφέ.
After having read the discussion below, I tried Ο άντρας θέλει έναν καφέ
It was rejected because of the έναν, which was corrected to ένα
"Καφές" is a masculine noun starting with κ. I think the accusative (αιτιατική) can only be "έναν καφέ" rather than "ένα καφέ".
Both έναν and ένα are correct for masculine accusative. However in this case you’re right, because the next word starts with κ the correct form would be έναν καφέ.