Translation:The boy has said nothing.
Why is "De jongen heeft niets gezegd'" wrong? When do you use 'niks' and when do you use 'niets'?
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.
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!
How would you say,"The boy has nothing to say" De jongen heeft niks om te zeggen?