Simply put, Walter does not know about Ruth's condition. The audience doesn't know until later, but Ruth is carrying the emotional and physical weight of a pregnancy. This is part of the reason why Walter asks what is wrong with her. The other part of this is that there is the obvious tension in the opening of the play regarding the arrival of the insurance check. This causes a noticeable tension in the air around the Younger home. The fact that Walter is asking Ruth what is wrong with her might also be reflective of Walter's own anxious state where he is more worried than anyone else about that check because he has more riding on it than anyone else in terms of his own hopes and dreams.