No such thing
Podobno. Nie ma czegoś takiego jak darmowy obiad.
"Bezpłatny" not accepted?
As well as "Obiad za darmo".
I'm not sure why, but this showed up as I was reviewing the colors part of the tree :/
The Duolingo algorithm likes to put a random sentence into an unrelated skill from time to time.
What about "Obiad gratis"?
Well, gratis in Polish usually means that the meal is free but only when you buy something in the first place (kind of tying agreement)
Wolny is free, as in liberated, unchained.
Why not kolacja?
Because "kolacja" is the evening meal, it's definitely not lunch.