Do I have to write a report if a player is sent off?

Yes, in one situation. Under World Rugby Laws 2026 (Law 6.27e), if your verbal report to the referee leads to a player being sent off, you submit a written report about the incident to the referee as soon as possible after the match, and the referee provides it to the match organiser.

Separately, the referee must give the match organiser a written report on any sent-off player's foul play as soon as possible.

Source: World Rugby Laws of the Game 2026, Law 6: https://passport.world.rugby/media/wnvjlxtb/2607en-world-rugby-laws-final-compressed.pdf