When I try to submit the exercise I get the error message "Nothing found. Add models to your Sketchfab Account and make sure they are published. Need an account? Sign up at Sketchfab.com" However I do have a sketchfab account, and the model is published.
I have another problems with submiting that one
When i select my blend file from the sketchfab and press "Submit" there are some error appear - "Your Submition not created" =(
It's been a while, so mby i can be wrong, but i think i solved that problem with simple action - just fill up all boxes: "what's it called?", "about your submission". You can write anythin, for example "what's it called" - retopology exercise, "about your submission" - it was a pain in the ass, but i finally complete retopology of the model!
And only after that you can select your sketchfab file and add submission
also mby it's CGC's server problem and you just need wait a while, and try again later.