Hi @rmarquez_marti, thanks for posting on the Forum and welcome to the SimScale Community 
It’s a bit tricky to pinpoint what the problem is exactly since we don’t have access to your CAD model. However, keep in mind that STL files and faceted geometry are usually not great for most simulation types as the mesh will fit onto surfaces.
Even then, you should be able to import the files, but if the STL file contains a very high number of faces it’s import will become slow and might lead to errors/crashes. What is the number of faces in your file?
Cheers
Igor