When can a coach verbally appeal a runner missing a base?

A coach can verbally appeal a runner missing a base only when the ball is dead because that is the point at which the game is paused and the umpires can address the appeal without ongoing play or any actions that might interfere with the situation. During this time, the coach can notify the umpire about the infraction and ask for a ruling on the appeal.

When the ball is alive, play is ongoing, and the umpires need to focus on the current action in the game, which would not allow for an appeal to be processed. If an appeal is made after a pitch has been thrown, that situational dynamic can confuse the unfolding play, as the focus should still be on the current play in motion. Therefore, the period when the ball is dead is the only time when a coach can make such appeals effectively.