Can a referee disqualify a goal scored when realises later the coach of the team scoring the goal was in the playing field?

Yes, a referee can disqualify a goal if it is later realized that the coach of the team that scored was on the playing field during the goal. According to the Laws of the Game, only players are allowed on the field during play, and the presence of a coach can be considered an infringement. The referee has the authority to nullify the goal and may also take further disciplinary action against the coach.