Typically STL files are not very cooperative with the mesher as it requires a well defined solid geometry to mesh. However do share the project link first like Darren has mentioned so we can better identify the problem.
Quick questions is the first and second geometry supposed to be that big? “sketchup” is over 200m across and duct_revit is over 2000m across.
Cheers.
Regards,
Barry
EDIT: I’ve converted the STL file to a solid file along with reducing the scale to something a little more realistic. It has meshed fine and you can see it under “mesh 3”.
I wouldn’t consider it fixed as it is just simply a bunch of triangulated surfaces due to the STL file format. I recommend obtaining or re-drafting the original CAD file and converting it to IGES/STEP/SolidWorks part file instead. It would be more robust and easier to work it as well.
For this case I imported it into SolidWorks and selected the option that would turn the faces into solid surfaces. Then just simply upload it and you can see what you get. The mesher also tends to not like oversize geometries especially for STL so do take note of that.