Translation:This is the man whose brother is my friend.
"Αυτός είναι ο άντρας του οποίου ο αδελφός είναι οφίλος μου." is one of the correct answers we have in the incubator. It should have been accepted. Of course, we can't see what you wrote or if there is an error. Please send a screenshot when this happens so we can be of more help. Often we find there are other errors. Even one letter from the wrong keyboard could cause the computer to reject a translation.