You are either trying to mesh the air above the surface or the ground below the surface. I'll assume you want to model the airflow.
The image you show is for an STL file that has six sides, the surface you want and five other sides you don't want.
In SpaceClaim, on the Facets tab, select all the facets on a side face and hit Delete. Do that for all four sides.
Now use the Separate button. You will end up with two open Facet surface bodies in the outline. Select the one that is not the surface you want and hit Delete. Now you have a single surface that represents the ground. Use the Check Facets button and repair any defects in this surface using the tools on the Facets tab.
On the Design tab, create a plane below the lowest point on the ground and sketch a rectangle that is smaller than the size of the ground. Pull that up to a height that represents the air domain.
Now you can use the ground surface to split the air block. This should have a reasonable chance of being meshed.
If you have difficulty meshing, you can use the smoothing tools on the Facets tab to make it easier to mesh.