Hi there. If you click on the error, you can see a description of the fault:
This means that the faces are overlapping.
Also, your model still has a sheet face, instead of a solid part. In general, I would recommend you use STEP or Parasolid format.
Best regards,
Fillia
Well, I’ve searched many tutorials on how to convert .stl to STEP but I’m struggling to succeed in doing that. I’ve tried FreeCAD, I have to try SolidWorks and Fusion 360, but the last one isn’t free…
You could try OnShape as well, it is cloud based, and free 