"Ni äter en måltid."
Translation:You eat a meal.
It is, when addressing an individual. "Ni" is how you would address multiple people or a plurality. Kind of like saying "you all". If you were talking to a dating/married couple - and you wanted to ask how they met, you would use "ni" to include both of them in the discussion. It's the "royal you", if you will.
Ni is more like vosotros, depending on from where the Spanish speaker is.
Duolingo is saying "You eat one meal" and using "one" as "en", instead of a. Unsure if it would accept a, but is it both?