Colloquially you might hear 'ingen leketøy', as I believe 'intet' is declining in use, but 'ikke et' would probably be the most common way of phrasing it. The purpose of this sentence is to teach 'intet', so while it's not an incorrect sentence, it's perhaps not the best sentence.
Based on the exclamation mark, I assumed there was strong emotion in the sentence and wrote, A flute is not a plaything! which was accepted. A post elsewhere in the forums said that tøy is often used as a suffix meaning "thing." A handy suffix to know as I assume it could be paired with many prefixes and used to mean any kind of thingamabob?