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Can anyone help me understand why this isn't working?
function arena_level_up_pl1F (hero)
MakeHeroInteractWithObject(heroname, "monster_arena1_pl1");
end;
Trigger(OBJECT_TOUCH_TRIGGER, "arena_level_up_pl1", "arena_level_up_pl1F");
You definitely need to replace heroname with hero. If that doesn't help and there are no errors in the object names, it's possible that you can't interact with the object monster_arena1_pl1, or that you can't use OBJECT_TOUCH_TRIGGER on arena_level_up_pl1.
Try enabling the console; you can find instructions online. It will be very helpful.