Can't make a solid group

So I want to 3d print this house and if I understand correctly, it needs to be a solid group to do that. I have edited it to be as simple as possibe and made sure there are no unnecessary lines or faces inside the house (as far as I know). When I try to use the “outer shell” tool, I only get an error message. I will upload the sketchup file, if any of you would be so kind as to take a look at it.

Catan only house.skp (13.3 MB)

Get rid of the internal faces and it should be good.

In order for a group or component to be considered solid, every edge must be shared by exactly two faces. No more and no less. With internal faces you have an edge shared by three or more faces.

By the way, it wouldn’t hurt to get into the habit of purging unused stuff from your models. In this case I reduced the file size from 13.3 Mb to 125 Kb. More than a 99% reduction.
Screenshot - 7_21_2020 , 2_25_10 PM

I deleted the faces in red on your picture and it worked. Thanks! Is there an easy way to make something more complicated, like the oscar I took from the 3d wherehouse, a solid?[Catan.skp - SketchUp Pro 2020

And btw, what is it that makes the file so large? I make a copy of the file and deleted everything but the house.

Solid Inspector is available at no charge but you could send the author a donation, cookies or beer seem to work well. For the time it saves, it would be worth paying for if your time has any value.

simply deleting content from the model space does not remove it from the file. Components remain in the In Model components as does materials and empty tags. Also styles you might have tried are kept in the In Model Styles collection. This can be useful in some cases. Think about inadvertently erasing an object instead of hiding it. (They are next to each other in the Context menu.) A group will be gone unless you catch it shortly afterward and undo back to before erasing. If you accidentally delete a component you can bring another instance of it in from In Model components.

That statue would add a fair amount to the file size.

This is a good example of the kind of problems that can arise from downloading components from the 3D Warehouse directly into your model, particularly when you don’t purge them if you aren’t going to use them. It is always better to download them into an empty model and then inspect them to see whether they are suitable for your purpose and to clean them up as needed.

Because they are triangulated surfaces, the Voldemort head has almost 60000 edges, and the Oscar statue has a bit over 12000. Though not vast by SketchUp standards, those are far larger than the edge counts in the house, and (as @DaveR noted) having even unused copies of them in the components collection will increase file size. The unused components also have a number of materials associated with them, which too contribute to file size.

Some of these components (again, notably the Voldemort head) are modeled at huge dimensions, so to use them in a model you would in any case want to shrink them down to real size.

Because it was not particularly well built, it will take a fair amount of work to make a solid from the Oscar statue. For starters, it is an assembly of nested groups. Although some 3d Printer slicers will accept a nested structure, SketchUp will not recognize it as a solid. So, you will have to explode the nested groups.

Then you will find that the various pieces have defects such as surface borders (unclosed holes between the exterior and interior) and internal face edges. There are so many that repairing them will be quite tedious. Most of them will need manual action because it is not clear how the surface was meant to be completed as a solid.