Why not "you've just made coffee" ?
Is it simply because of the contraction, or is it actually wrong to use a present perfect?
That answer should be accepted. If it happens again, could you please take a screen shot and file a bug report? There may be a problem with the system recognizing contractions.
The system tends to accept possessive contactions, but less often verb contractions
Why is it de faire and not just faire?
The fixed expression is "venez de" + the infinitive verb. Venir (conjugated) + de + the infinitive means "just did" something or something just happened. This is the near past. You can read about it in the Tips and Notes for this skill.
Just realised that the verb 'venir' is 'venir de' and that dictates the usage of 'de' or otherwise.
What is difference for "going to make coffee" vs "just made coffee"
I'm going to make coffee. → Je vais faire du café.
I just made coffee. → Je viens de faire du café.
Use aller for future events and venir de for near past ones.