The front view looked fairly straightforward to fix…
BUT the back has several nested groups etc, making it non-solid.
These need exploding.
Then they’ll intersect the remaining volumes, making it non-solid again.
There’s a lot to fix.
Also you have modeled it at a small [real] size - SketchUp cannot cope with creating edges <1/1000" long as it sees both ends as coincident, then reliant faces are also missed.
Model it say x10 bigger and rescale down to the desire size at the very end.
Small geometry can exist, but it cannot be created.
To recap on solidity…
A ‘manifold solid’ is a group or component-instance which contains only ‘geometry’ [edges and faces].
This means there can be no nested groups of component-instances, or other non-geometry entities, inside the ‘container’.
Also every edge must support exactly two faces - no fewer and no more.
So, no faceless-edges, no edges forming a perimeter to a hole in a surface or a ‘shelf/flap/ledge’ form, no internal-partition faces [which will have three or more faces], no otherwise seemingly solid volumes sharing an edge [e.g. two cubes with one common edge - where that edge has four faces].
Also note how all faces should be oriented ‘outwards’ - you should see no back-face materials when viewing in Monochrome mode - usually blue-gray [depending on the Style] - the front face material is usually off-white.
The context-menu can be used to ‘reverse’ badly oriented selected faces [i.e. it flips them over so the back/fronts swap], but if the object is a true solid then the context-menu ‘orient’ option with one selected face that is already properly oriented will orient all connected geometry appropriately to match.
Also note that if the container’s Entity Info says “Solid”, then it probably is 3d-printable - BUT it is possible to make a form that reports as solid which cannot exist in the real world - typically this has geometry which loops back into itself, but does not intersect with any faces through which it passes, like magician passing his arm through an imperforate brick-wall - in reality it’d be impossible.
To check for this… select all of the ‘Solid’ object’s geometry [inside an Edit] and use context-menu > Intersect > with Context…
Now if you leave the Edit mode and Entity Info still says it’s still a ‘Solid’ then it really is ! But if it no longer reports that then parts of the geometry have now intersected and make things like partition-faces which preclude solidity.
To fix the various issues which lead to non-Solid reports you need to Edit the container, explode any nested entities, and delete the unwanted internal faces, or flaps; also heal holes by overdrawing edges to complete the faces etc.
Using X-ray mode and temporary section cuts will assist in seeing what needs doing.
Thomthom’s Solid-Inspector shows issues.
His Solid-Inspector2 attempts to fix them.
My own SolidSolver also tries to fix issues - including self-intersecting volumes and the common issues like holes and internal faces…
However, no algorithms can beat your eyes and brains in fixing your own solids - it’s easy to make a form that is un-fixable using a plugin…
Save those for fixing the few issues you can’t fathom, after you have manually done all you can - the potential solutions increase exponentially as you add more errors…