"The apple is not falling onto the chimney, but onto the fence."
Translation:Nem a kéményre esik az alma, hanem a kerítésre.
In this case the word order is important, because your sentence emphasises the apple "az alma" or falling "esik". In the original sentence the main emphasis is on the chimney "kémény", and for this reason we use the word order of the original sentence.
It's incorrect, because "esik" is negated instead of "a kéményre". If we only had the first part of Duo's sentence, your translation would be perfectly fine. But "hanem a kerítésre" makes it clear what has to be negated: the phrase that contradicts it, which is "a kéményre" here. So "nem" has to come directly before "a kéményre".