Translation:I am not an actor, but a reporter.
The only corrext option would have been " i am not an actor, a reporter." That doesnt make any sense.
I wrote "I am not an actor, but rather a reporter," and was marked wrong. Is there an error?
but rather, but instead sound fine to me as a translation of hanem, but neither is often accepted yet. Report them so that they can be added if you would naturally use one of them.
That sounds wrong to me -- you are saying "I'm not an ACTOR, but a REPORTER", i.e. comparing the professions, so (a) the thing you are comparing should be in front of the verb, in the focus position, and (b) it should be after nem.
So Nem színész vagyok, hanem riporter. (Duo's word order) seems best to me.
Yours seems to be something like "I AM NOT an actor, but a REPORTER", which makes little sense to me.