I have the same bug. Failed to have an enemy rival. I joined the cheerleading team and went to the festival, at this time there is an opportunity to argue with rival. You have to quarrel very quickly, as the action disappears after the festival. But even if I manage to recognize the Sim as an enemy, the goal does not end. I removed mods and it didn't help.
The rival sim may have aged to young adult. Try leaving the extracurricular activity and entering another one, normally as soon as the active sim enters an extracurricular activity from the High School Years pack, the game already adds the rival sim in the relationships panel.
