"Нет проблема" was marked incorrect. Should this be accepted as an answer? I only ask because I am sure I have heard native speakers using this in their spoken language. Thanks.
«Нет проблема» looks definitely ungrammatical to me. Are you sure the native speakers you've heard weren't saying «Не проблема» '[It is] not a problem'?
Извини, но "нет проблема" - это некорректно. "Без проблем" - правильно.
Проблема is feminine, so since нет requires genitive, you'd need проблемы (gen. singular) or проблем (gen. plural). I think szeraja zhaba's suggestion likely explains what you heard.
So, "Нет проблемы!" should be accepted, yes? That is what I answered, and DL corrected my answer to "Нет проблем!"
As a literal translation I suppose it's not incorrect, though I think I've usually heard people say 'нет проблем!' or 'без проблем'.