Bonjour Martina,

Thank you for your comment! As mentioned in the lesson, 'de l'après-midi' is not used with the equivalent of 6 PM. From 6 PM onwards, you can say 'six heures du soir' (12 hour clock ; as the evening "starts" from 6 PM) or dix-huit heures (24 hour clock)

I hope this is helpful.

Bonne journée !