"The one who cannot be quiet rarely knows how to speak well."
"Celui qui ne peut pas être tranquille sait rarement bien parler" What is wrong with this? Is it not a more exact translation from the English?
It's correct all right. But the meaning is not the same. You have an pretty clear idea here that the sentence means that is if you can't quit speaking, you will not speak well, you will just speak too much. I know it seems unfair, because you're right with your sentence, but sometimes duolingo wants you to stick with what he gives you.