Translation:I don't eat at noon.
As Chinese, I understand the sentence :
"I don't eat lunch" (well, I know more people would rather skip breakfast)
"I don't eat rice for lunch" because I eat noodles or whatever, except rice
The 1st sentence seems to be more suitable since even I serve noodles for dinner, I say "吃饭啦" (let's eat rice) which means let's have meal.
不 isn't used for the past tense, you'll see it for the present and future tense. In the past tense, we'd generally say 我中午没有吃饭. However, if you wanted to say you did eat, then you could add 了: 我中午吃饭了 (I ate at noon). In negative sentences, we use 没有 or just 没 instead of 了.
Too many mistakes in the Duolingo Chinese module - "吃饭" is a general term for "to eat", even Google Translate knows this (or anyone who has taken a 101). "米饭" is the more specific term for rice. Everyone, don't rely on Duolingo if you want to actually learn this language, I see errors every single time I go through something.