Translation:I want some tea.
It has showed up before, and it's actually an adjective. I'm not too sure about the exact definition, but from the way it's been used so far, I would say it's equivalent to "desired" or "desirable". So こうちゃは欲しいです would literally translate to something like "(black) tea is desired", which can be converted to "I want (desire) some tea."
'I would like black tea' was not accepted. I guess duolingo must find out if we know the meaning of hoshii, ie the literal translation. But what is the politeness level of this japanese phrase? If it is quite polite, then more polite translations of this should be allowed - or maybe this is a philosophical discussion :)