Thank you all for letting us know, and I'm sorry about the trouble! We're looking into it right now and will let you know when the issue is fixed!
Thank you all for reporting this! After some investigation, we found the problem and pushed a fix. You should be able to complete the course now by creating a new branch. If you are unable to move forward after creating the branch, please drop me a note in this thread and I will manually move it forward.
I think that after the fix it jumped directly to 'Step 6: Open a pull request' automatically without me having yet completed 'Step 5: Commit a file'.