Mesh generation in the stl file

I tried to generate the mesh at the stl file, but I have a problem.
When I generate the mesh, there are wholes in places like following picture.


The overall shape is as follows.

The thickness of the walls is 0.1mm. Do I need to increase the thickness?

And I don’t know if it’s because of this, but even if I create a mesh, I only create a boundary mesh and no inner mesh.