Additional shapes when 3D print (slice)

Hi! I have a problem - when I export an STL from SketchUp and import it to Ender Print (slicer), it looks ok, but when I slice the file it adds extra shapes. After 40+ different tries still no luck.

Can someone help to understand the reson?
storefront1.skp (1.6 MB)

You need to fix the problems in your geometry.

Hide the top face so you can see inside. delete any internal faces and edges you find. Every edge in the object needs to be shared by exactly two faces. No fewer and no more.

Is this for a model railroad layout or something?

Compare this version of your model with the way yous is built. Note that there are no internal faces or face holes.
storefront1 solid.skp (189.3 KB)

BY the way, I see your model contains all sort of unused components. It’s possible that the .stl file you exported contains geometry from them. Best is to purge unused stuff from your model. Notice the file size reduction.

Great advice @DaveR, hadn’t had this problem before. And thank you for your input, I really appreciate it!

“Is this for a model railroad layout or something?”

Sort of, back in my childhood there used to be movie or series called “The Borrowers” - small people living in walls, so I am making a miniature city under the kitchen cabinets for my kid to create some magic/mystery.