This sentence shows why it would be so helpful to have a native Greek speaker on board. I think it’s wrong, but I’m not sure. The use of ‘appointment’ in English suggests something like a doctor’s appointment. But I think διορισμός in Greek refers exclusively - certainly mostly - to the use of ‘appointment’ to mean ‘designation to a post’, e.g., ‘she turned down an appointment to the diplomatic service’. None of the Greek-Greek dictionaries that I have consulted seem to suggest διορισμός as a possibility for ‘appointment to see a doctor’, etc. And the word is related to the verb διορίζω, which is generally translated ‘I appoint’ [someone to something]. Would welcome help on this point!
A postscript: I suppose you could interpret the Greek as “I meant to cancel your appointment [e.g., to the diplomatic service] today”, but I don’t think that’s how it would usually be read by an English speaker.