"a small television in a big living room"
Translation:تِلْفاز صَغير في صالون كَبير
That's because it's not a sentence! If you were doing this exercise on your smartphone, the instruction will have been "translate this sentence". I've been protesting at this misuse of "sentence" for some time now. But Duolingo continues to call a phrase a sentence. At least, in the web version, the instruction is "write this in English", which is much better. Why should they commit themselves as to whether it's a phrase or a sentence? That's for us to work out.