Is this a parting phrase?
Is usiku njema wrong? If so why?
Yes, because njema goes with N class nouns only.
how is that possible i am native speaker and i didn't know that
I am told by native speakers that this is not a good thing to say because it is wishing someone a good 'romantic' evening.
I have a question... What does Have a and what does good night mean in that? Or is it all one phrase and cant be saud any other way?
It is a manner of wishing that someone have a good night and is more familiar.