Your rejections are becoming odious and in many cases incorrect duolingo! Kindly alter you ACCEPTANCE PROTOCOL! Both answers mean the same thing. All your rejections do is cause students to find what answer suits your protocol rather than study to learn the language. Your rejections create a confusion is the mind of the student and this can create more error!
this is a spacious kitchen >هذا مطبخ واسع
this spacious kitchen >هذا المطبخ الواسع
this kitchen is spacious >هذا المطبخ واسع
this is the spacious kitchen >هذا هو المطبخ الواسع
This kitchen is just as correct a statement duolingo and should be accepted. It is possible this statement is used more times than the statement you accept as correct!