I wrote "de van hó fent" and was marked correctly...

Warning: I live in Budapest, and can have basic conversations, but my grammar is terrible! So, I would trust the given translation as being the better option!

Are you trying to translate "there is" with "ott"?

"there is", in English, indicates the existence of something, but does not specify a location -- the fact that the first word is spelled like the word "there" which does specify a location (~ "in that place") often confuses English learners who are learning other languages.

If you want to specify that something exists in that place, it would have to be "there is ... there".

Here, there is no "there" in the sentence, and so ott doesn't really make sense to me.

Also "Hanem ott hó van fent" is wrong?