We don't even say «μαγειρέψει πρωινό»! Instead: Αυτοί είχαν ετοιμάσει πρωινό, είχαν φτιάξει πρωινό.
Obviously it's not an Irish fry-up, eggs, rashers, sausages and black pudding. Yep I know my Greek friends are appalled at the mere idea!
"They had cooked breakfast." is one of the accepted translations. If that's what you wrote and it was reject could you send a screenshot so we can determine where the problem lies.
Sorry, failed to take a screen shot - the clock ran down on the timed practise while I was writing here. I'll look out for it next time.