There is no such thing as an ace in softball; you might be thinking of volleyball or tennis.
If the player is unable to hit the serve then it is an ace. But if the player hits the ball but does not return it, then it is a forced error.
That is called an ace.
An ace is when the server serves in the opposite diagonal front box and the returner fails to return it not even touch it
In tennis when the opposing player is unable to return the first serve, this is known as an ace. An ace is also a hole in one in golf, and obviously card games.
There are two types of serve that a player cannot return, one is an Ace and one is just called Unreturnable. An Ace is when the returner cannot get their racket to the ball at all. An Unreturnable is when the returner touches the ball, but cannot return it within the court.
Full Ace tennis