I had the same error. I tried to build the Landscape Spline Meshes but it still didn’t work. so I tried to build another map and check the cause. everything works fine until I use the landmass brush or water.
It hurts that it doesn’t work but they do mention that it’s in beta and experimental.
I hope someone can find the solution someday before the stable releases or they will come soon.
In the meantime be careful with these tools, I wasted all my time creating the lanscape and looking for a solution.