Translations:FreeCAD and Mesh Import/2/en

"cannot convert because shape is not a shell"
Well, your shell seems to have errors, maybe it is not closed (has holes). Since the capabilities of the mesh workbench in FreeCAD are a little limited at the moment, you might want to try examining and repairing of your model with third-party software. After doing that, you may try importing and converting of your model again.