From the Sketchfab blog:
We’re glad to announce that we now fully support Blender 2.90+. While we had been closely following the 2.90 beta releases, the final release contained some small file format incompatibilities with 2.83 that caused processing errors on our end in some specific cases. This has now been resolved, and both 2.83 and 2.90+ files will now work as expected 🎉.
Discover our Blender add-on
Sketchfab supports direct .blend uploads via drag and drop on the website, but a far more convenient way to work is by installing our Blender add-on. The add-on makes uploading to Sketchfab quicker and easier by packing all required assets and setting all the model properties for you before uploading, directly from inside Blender. It will also let you search and import over to 450,000 Creative Commons models from our community library.