"The black clock is not there."
Translation:A fekete óra nem ott van.
Yes, they are both correct translations of the English sentence (other than the typo). But the two Hungarian sentences are different.
"Nincs ott" simply means "it isn't there", as an answer to "is it there?".
"Nem ott van" implies that it is somewhere else. It is not there but instead somewhere else. The "nem" is negating the "ott", meaning "not there".
So, "it is not" versus "not there". The English translation is the same for both, with different emphasis.
My best guess would be that the word order is different so it is emphasizing the 'not here' part rather than 'what' is not there. As far as I know it's not technically wrong, but the emphasis should be on the object you're asking about, therefore it should be at the beginning of the sentence... but I've only been following this program for a week, so I could be off base.
If you think that your answer should be accepted then report it and at least they'll hopefully take a look at it.