Create a BoundaryDiscretizeRegion?

Hi
Sort of confused by this error (highlighted in yellow) below. Did I not created a valid region with RegionUnion ? I can see it, it is indeed a solid with closed boundaries. Why would any command (I’ve tried all of them) seeking a boundary come back with an error ?

I would ultimately like to create a properly meshed solid object from the boundary–Any ideas or work arounds for the situation below (and attached) ?

Kind regards,

John

What is the “boundary” of a surface in 3D space? BoundaryDiscretizeRegion is meant to work with solids:

BoundaryDiscretizeRegion[Sphere[]]
BoundaryDiscretizeRegion[Ball[]]