I would guess, in this exercise "waiter" is not accepted, because it might mean you didn't get that офіціантка is specifically a female waiter, a waitress. Since the exercise wants to test your knowledge, that answer is not accepted because it's unclear if you got that part or not.
Is it correct to say "waiter" for a woman?
Is it correct to say "female waiter"? Or is it not a thing because, you know, the word "waitress" exists :D
If you answered my questions with "no", then why did you write "but my answer wasn't accepted"? If the answer to my questions is "no", then it shouldn't be accepted indeed.