Since you hardcoded #trivia in everywhere else and also havent shown us what Event this goes off on, im assuming the problem is its not a channel event, and thus $chan has no value, so try...
if (%cryptos.zelfpunt >= 500) && ($nick !isvoice #trivia) { msg #trivia Opgelet: $nick heeft meer dan 500 punten. Extra modus wordt toegevoegt | mode #trivia +v $nick }
PS: i removed the /, you dont need them inside scripts!