It gave me wrong when I wrote "She eats at lunch" why ? the answer is "She eats at noon". What's the difference ?

As far as I know, "lunch" is the actual food, not the time. "noon" is time.

It's not the same thing. She could be busy and only have time for a single bar of chocolate at noon for example and have her lunch at 3pm. She would still eat something at noon, but not her lunch.

Mittag means noon. While Mittagessen is lunch it doesnt necessarily mean lunch is always at noon. Here they are specifying what time does eat.

What does "eat through noon" mean?

Noon for time of the day, like in the morning,at night etc

I tried "she eats in the afternoon" and it considered it wrong.

"afternoon" - "Nachmittag"