Convert to solid mesh gives me multiple surfaces

[ischarf2]

I am running into some issues with analysis between two surfaces (calculating a volume). What I'd like to do is extrude from a created plane down to a surface object. It overloads the computer when I have the 'standard' # of faces, so I've tried to import mesh -> modify -> reduce/simplify. When I try to convert this lower face # mesh to a solid, it creates surfaces (the orange icon). 

Subsequent to this, I can't use the extrude function to then create an object. 

For reference, this is what I am aiming for - but it is very slow right now given the # of triangles: 

[TrippyLighting]

Converting triangulated meshes directly into solid bodes, (almost) regardless of mesh resolution is such a bad idea that the current director of software development for the Fusion 360 team wish they had not implemented the feature.

Windows and MacOS have built-in features to take screenshots. If you want a post to be taken seriously, please Do not take screenshots (or even worse scree recordings) of a computer screen with a phone.

Embedding such a photo into a PDF and attaching it instead of directly embedding the photo in the post is adding insult to injury!!!