Why is "De jongen heeft niets gezegd'" wrong? When do you use 'niks' and when do you use 'niets'?
There is no difference between 'niets' and 'niks' ('niks' is a bit more informal in my opinion). So your sentence should be correct too. I hope you reported it!
In a "type what you hear" exercise you have to type the exact word that is said by the voice, so then you can't replace a word by a synonym.
But you can't really make out "the exact word that is said by the voice", most of the time.
How would you say,"The boy has nothing to say" De jongen heeft niks om te zeggen?
This is an unnatural English sentence. I feel "The boy hasn't said anything" is more common to say, if one were to express this idea.
Is "The boy said nothing" not an appropriate English translation here?