As a simple example for applying stl-files I took "cow" out of MMA example data. I'm able to discretize the Graphic without problems

kuh = ExampleData[{"Geometry3D", "Cow"}]
mesh=DiscretizeGraphics[kuh,MeshCellStyle -> {{1, All} -> Red}] (* MeshRegion *)

to get an stl-like triangle surface, which seems to be ok

ConstantRegionQ[mesh]
(*True*)    

for further meshing, but my attempt to create a volumemesh fails

Needs["NDSolve`FEM`"]
ToElementMesh[RegionBoundary[mesh]
(*$Failed*)
 

What's wrong with my attempt? Thanks!