How about sharing the .skp file instead? It’ll make it easier to guide you in correcting the file.
The common reasons that models aren’t printable are reversed faces (blue back faces should be toward the inside of the volume), internal faces, and holes in faces.
Based on the .stl, there are a number of problems with the file.
Here I’ve reworked Ava’s model to make it a solid. Compare this one to hers in SketchUp and try the .stl file in your slicer. I modeled in inches. Not sure what she used for units. Avas Pencil Holder.skp (102.9 KB) Avas Pencil Holder.stl (105.2 KB)
Notice that I recessed the text a bit so it would show up in the print.