If you make a group of the geometry that you’d like to be a solid, then its Entity Info contains the word ‘Solid Group’ in the top bar - otherwise it says ‘Group’.
Solid-Inspectore will highlight problem geometry.
Even without using it I can see several issues.
All of the faces in the group are reversed.
Edit it, select one face and use the context-menu to ‘reverse’ it, then immediately ‘orient’ and ll connected faces will flip to match it.
There is an upstanding part [a ‘box’] which you have placed passing through the ‘roof’. If if it did not intersect with the main part at all then it’d report as a solid BUT 3d printing would probably fail. As it intersects with the roof on just one edge then it is reported as non-solid. It must ‘intersect’ it with the roof [context-menu on selection], so there are additional edges added where it passes through the roof, and then the unwanted internal parts must be deleted so your group is a single-skin == solid.
A solid must contain only faces and edges [all groups etc exploded].
Every edge must has TWO faces.
So, no faceless edges, no ledges/shelves/flaps/holes with an edge with one face.
No internal partitions where some edges will have three or more faces.
No overlaid faces which are all but coplanar but some of the edges will have too many faces.
You also have a few issues with edges and thin-side flaps etc - Solid-Inspector will show those - erase them…
Also switch off your units length-snapping - it makes adjusting thing harder than it needs to be…