On previous translation in this module, DL used "είναι ένας", so, can you also use: "Αυτός είναι ένας σερβιτόρος"?
Yes, but αυτός & ένας can be ommited.
How to say "they are waiters"?
Αυτοί είναι σερβιτόροι.
I don't see that I was wrong
We don't know what your answer was though.
Next time take a screenshot so you can show to the moderators and they'll hopefully figure out what was wrong. Or at least copy and paste your answer here.
Why "A waiter", when we translate σερβιτόρος" without article???
Because English grammar requires the indefinite article here (before a singular countable noun).
Αυτός είναι σερβιτόρος
Αυτός είναι ο καπετάνιος
Why the article was used in one sentence but not in the other?
It's the same difference as in 'he is a waiter' and 'he is the captain'.