You either separate "nincs" and "sem", or put them beside each other, in which case they merge into "sincs". It should've probably been accepted in that other sentence too.

You cannot use nem and sem together. You could say "Ő sem itt van." which in English would be the same sentence: "He is not here, either." However in Hungarian "Ő sincs itt" and "Ő sem itt van" mean two different things. Ő sincs itt: here the emphasis is on the lack of his existence. You are saying that among other people he is one who is not here either. Ő sem itt van: here the emphasis is on "here". It means that he is somewhere else, along with other people. Same goes for "Ő nincs itt." and "Ő nem itt van". The first simply states he is not here, whereas the second one suggests he is somewhere else.

"sem" means that not only others, but even the subject of the sentence does not satisfy the condition of the sentence.

"not ..., either"

What exactly did you enter?

Are you sure that what you pasted was presented as “the correct solution” or as two possible solutions?

(I’m just asking because a lot of people submit such double translations instead of picking one of them.)