>>Do you get any specific error message with sketchfab?
No, its regular “Error 20” message. The platform doesn’t accept Blender 2.8x files yet (same goes for Sketchfab Import/Export add-on).
I’ve tried some ways with exporting different formats (didn’t help) and finally came up with the only solution that worked:
Delete 2.80 / Install 2.79 / Do that lesson again / Upload the result to the Sketchfab.
