Hi Kimji- It’s hard to say without knowing a bit more - stl files are 3d geometry, they are meshes, and FE analysis is on meshes, albeit very likely not on the result of a scan - perhaps a more orderly mesh is needed, or to make sure it is closed, or the analysis software wants to make its own meshes from a solid?
I would suggesting checking the model using the Mesh Repair Tool. 3D scan data can often contain errors but can be easily cleaned using the Mesh Repair Tool. It is best to make the repair on the original stl data before you reduce the mesh.