It may simply be that d’après’midi (or even d’après-midi) is not correct as it should be de l’après-midi.

However, it could also be because you used ‘trente’ instead of “et demie”. This is a matter of usage - when using the 12 hour clock, it is traditional to use it in its standard manner, just as it is standard to use the 24 hour clock without saying quart/demie etc

This means using et quart, et demie, moins le quart when on the quarter hour. Also requires using the right form for minutes past or before the hour: for example “deux heures dix” or “deux heures moins dix”. 

And, from experience, you are likely to get a gentle correction in the form of having it repeated to you correctly if you don’t !