For starters, there are a couple of basic issues with the model.
- You have a large number of unused components that are bloating the model. If you do Model Info->Statistics->Purge Unused you will see the file size decrease noticeably.
- The unused components are also not using layers properly. They associate their edges and faces with layers other than Layer0. All edges and faces should be drawn with Layer0 active and left there. Only Groups, Components, and some non-geometry items such as dimensions should use other layers.
That said, for starters if you plan to export as stl you should first make your object a group or component so that you can test whether it is what SketchUp considers a “solid”. Most printers won’t correctly process a SketchUp model unless it is a solid. When it is made a component, the Solid Inspector 2 extension will show you the errors @DaveR illustrated.
Some of these SI2 can fix on its own, so your first step should be to ask it to “Fix All”. There will still be some errors left that you have to repair manually. Several of them result from small gaps in the arcs that define a couple of the holes in your object:
If you use the line tool to fill in those little gaps and then run SI2 again it will find a large number of issues but will be able to fix all of them!