The hungarian answer seems to be as "the big green crab is THERE, where the blue turtle is.
A nagy zöld rák van, ahol a kék tekn("o)s was rejected. Is it wrong?
Yes, it is wrong. You can do this in English but not in Hungarian.
Why isn't there a form of "to be"/van in the second half of the sentence. Does "ahol" cover that?
If you have connected clauses, you don't need to repeat the verb in the second clause if it's the same as in the first clause. But it's no mistake to include it either.