What should a referee do if a defender illegally enters the penalty area during a penalty kick and the ball is saved by the goalkeeper?

When a defender illegally enters the penalty area during a penalty kick and the ball is saved by the goalkeeper, the appropriate action for the referee is to stop the play and retake the penalty kick. This is because the Laws of the Game specify that all players must remain outside the penalty area until the kick is taken. If a defender encroaches and the kick is not successful (for example, if the ball is saved), the action of encroachment has compromised the penalty process, thereby necessitating a retake of the kick to ensure fairness and adherence to the rules.

In cases where a player enters the area illegally, the Laws protect the integrity of the penalty-taking process. This is also to prevent any strategic advantage gained by the defender's anticipation of the kick, ensuring that the attacking team has a fair opportunity to score. The retake of the penalty kick serves to maintain the integrity of the game and uphold the rules governing penalty situations.