Can't add player
There's no age restrictions on Mature games - does the player definitely exist, as in, they weren't banned as a double account or anything between sending the RTJ and you trying to add them? I assume you're sure you haven't banned or already added them (including as a Lurker, which don't show up on Cast Lists/the 'add player' dropdown) by accident.
Worst case, you can add players by username search, so since the name's on the RTJ, you could look them up, add them and then PM them to explain. Weird glitch.