"They see the waitress." is quite rightly "De ser servitricen." which is a slightly more old-fashioned way of saying it. The more modern, and thus more correct way would be to use "De ser tjeneren."
Why for male waiter duolingo use "tjeneren" and for female it is "servitrice"?
Makes sense. Servitrice sounds like servent
shouldn't they accept "tjenerinden"?
They should. But i have never heard anyone use that word