It said obiad was lunch previously
read tips and notes food1.
Short answer- Obiad is a big meal eaten in the afternoon, Kolacja is a meal eaten in the evening.
English dinner is both depending on the time you eat it, but according to most people complaining on Duolingo it is kolacja.
What is wrong about "ja jem kolacja"?
wrong form of noun "kolacja". It should be accusative "kolację"