Do you understand what the popup says?
It mentions that you have created a recurring appointment and it is asking if you want to alter this single instance or the entire list of appointments you have created.
I have the impression you are not aware of the fact that the appointment you have created, is recurring. So I'd advise you to check if your appointment should be recurring or not. If not, make it as single appointment. If yes, then I'm afraid there's no way to avoid that popup (which is very useful, by the way).