When I complete close a surface, Sketchup fillis it in the surface as expected. I hit C trl-s to save and the line closing the surface is lost. I am running Sketchup 2017 on Windows 10.
The geometry seems to be pretty messy. First thing I would do is reset the axes and move the model closer to the origin. There’s no need for it to be so far away.
I’ll freely admit I’m no Sketchup expert! I’m just working my way through using no extensions and learning as I go.
My first pass was 3D printed successfully, but the curves had too many sharp bends. I corrected the bends, but am having trouble now filling in the side areas . The top surface is a dome and the top edge is a curve so nothing on this is a straight line or plane.
I’ll give your suggestions a try and see if that helps.
Work with your model centered on the origin and keep it horizontal. When I model for 3D printing I set units to meters and model as if millimeters are meters. That helps to avoid issues with small faces.