"this is a very sad news " was not accepted (even there is "bir" in the sentence). should i report it ? (i am not native english speaker)
No - "news" is uncountable in English, so "a news" does not work.
But you can use "a" in defining the news.
Writing "piece of news" would work and would get round the issue of "news" being uncountable in English.
For me it does not work. I reported it.
Why is "This news is very sad" wrong?
If you pay attention to what goes on either side of the "is" in English, you can mirror that in Turkish.
This <==> very sad news.
Bu <==> çok üzücü bir haber.
This news <==> very sad.
Bu haber <==> çok üzücü.