When I open it "errors" says there's a parent part-copy missing.
I seldom work with surfaces so I'm wondering how does one turn such a part into a solid?
I show non-stitched edges circled in red. Most likely didn't stitch because of a gap.
Used "Show Non-stiched Edges".
Again, if I may: how does one turn such a part into a solid?
You are absolutely right that we miss the "make base feature of closed surface.
I also can't find a hole or missing surface when stitching and I think the body is closed.
There was one - not elegant but working - way I found.
Create a protrusion somewhere away of Your body followed by a boolean add and everthing together will form a solid body
A following cut can take away the original protrusion.
But it might be not the proper way I would like to say!