Modeling error: nothing has stitched

[LPFchan]

Hey everyone, Sorry to bother and ask this kind of basic help...

but I really can't seem to stitch this model into one solid body!

I've tried zooming in really close into the model, have set Max Gap as Stitch Tolerance, etc..

It would be grateful if anyone could help me learn what's the issue here.

(Sorry in advance for such a messy project file)

[TrippyLighting]

Of course this isn't going to stitch!

The shell is completely enclosing a space and can be stitched to form a solid body:

The center "thing" is not only does not enclose a volume, but it also has internal surfaces that make it a non-manifold. It stitches, but obviously won't create a solid body.

On top of that it is also imprecise, which is why it why other tools won't work,

[TrippyLighting]

I was able to identify the issue being imprecise modeling in several areas. Once done properly boundary fill can be used  to create a solid model.What I mean by "properly is that I deleted some surfaces, extended some, re-extruded others, stitched and trimmed them .

I'd redo this model from scratch if I were yo and pay close attention to modeling practices.