Currently sketchfab does not accept blender 2.8 files. Try exporting as GLTF 2.0. Several of the other options would work as well (obj, dae, fbx) but GLTF is the newest, best, and open standard.
You can also try this unofficial exporter/uploader. Follow the installation and usage instructions on this page.
https://github.com/thenewcoder/unofficial-sketchfab-exporter