How is "there is a park on the map" incorrect?
How do we know to use "the" on stead of "a" or vice versa?
Korean doesnt have "the" and "a", its understood from context. Duo doesnt give us context, so you can use either. Flag it.
It should be 지도에 공원있습니다
"A park is on the map" should be also accepted.
Use the flag
Why not "There is a map on the park"?
Because it says "지도에", that means "on the map". The "~에" indicates the place where something is. "On the park" would be "공원에".