I am having this same issue
Uploaded video here: - YouTube
I can spawn the trees on BSP geometry and also on a static mesh
But I cannot spawn trees on the most important thing: Landscapes
So in the video one terrain lets me paint rocks on it while the other does not which is also very strange. They are exactly the same terrain.
But the main issue is that none of my terrains allow any trees to spawn when I hit “resimulate” in the details of the spawner volumes.
When “resimulate” is hit it give the error: Unable to spawn instances. Ensure a large enough surface exists within the volume.