I wrote " My friend doesn't read an uninteresting newspaper." And it's not accepted.
Chances are you had a typo. "My friend doesn't read an uninteresting newspaper." is an accepted translation.
Isn't 'a' redundant? The sentence can refer to all boring newspapers in general
I guess for now they just want an exact translation before you slack off a little, probably so you can learn the basics.
I put "My friend doesn't read boring newspapers" and it was marked as correct, with a typo. I understand the plural in Korean is often assumed based on context and I thought "newspapers" sounded more natural than "a newspaper" in this sentence, but out of curiosity, would singular have been more accurate or was my answer fine?