Translation:Boys are playing football on the grass.
Here are some examples from a Google search for the phrase. "How many hours have your kids played soccer barefoot in the back yard in the grass?" "Here are the great benefits of playing in the grass versus a harder surface like concrete." "...snapping photographs of two top professional athletes who were nudging soccer balls around in the grass."
The first two sound like we're talking about taller grass :D In both cases I feel like I would say "у траві" in Ukrainian.
The second one is irrelevant because the ball is being nudged in the grass.
So far there's only one report asking for it... Maybe that's why I'm not convinced, no idea :') Like, I feel like nobody else really wants this translation and they're all fine with translating "на" as "on" :D
I would think it means "in" grass clippings or in very high (prairie) grass. "On the grass" would be the far more likely scenario. Or even, "Boys are playing football on grass" (said to differentiate it from playing on asphalt or turf) would be another sentence used in another context.
This is a strange thing to say in English. We would say either "The boys are playing..." or "There are boys playing..." but not "Boys are...".
Some English in this course appears to be constructed from "rules", beloved of overseas teachers of English but which are incomplete and which result in weird English like the approved answer here.