Student or Learner
I have make an appointment with Billy on 21 April 11:00. Please advise to me if this time is not ok/fit/available for you.
yes, it is.
The person could answer:
The suggested date suits us (well).
You can also say 'Please let me know if this time is not convenient' which is more formal.
But I would say 'I need to make an appointment with Billy ....'
saying I have made an appointment ' means that you have gone ahead and made the appointment, and it has to be cancelled if it does not suit..But it is not incorrect.
Zoe - it is not clear WHO you are talking to.
You say you must make an appointment with Billy. Then you ask someone if that time and date is OK.
Are you talking to a 3rd party?