"The brother sleeps at home."
Translation:Frater domi dormit.
To an extent. "Free" word order is not quite as free as people think it is, and there is always an unmarked default order, with other orders serving to emphasize different things.
And having no context from which we can determine emphasis, Duolingo should be more flexible in the word order it accepts.
The context is in the prompt. "The brother sleeps at home" is unmarked.
If the prompt were "It is the brother who sleeps at home", or "Home is where the brother sleeps", or "Sleeping is what the brother does at home", that would suggest a different word order in the Latin.
Those sentences have different meanings than "The brother sleeps at home." and would probably use different grammatical structures in Latin. Latin word order can indicate emphasis, but how it does so can vary widely. Conversely, English speech would typically communicate this through the weight given to a specific word: "The brother sleeps at home." / "The brother sleeps at home." In writing, we'd need more context to know what the emphasis is. "The parents cook at home. The brothers sleep at home." (emphasis on 'sleep') vs "The brothers sleep at home. The dog sleeps in the alley." (emphasis on 'at home')
But you can still technically order this in a different way than what they force you to, so he's not entirely wrong.