I think the Coursera grader might be glitching. I’ve seen similar issues across different courses. What worked for me was restarting the kernel, running all cells again, then saving and resubmitting. You may want to try that, as it solved the problem for me after a few attempts (more than 5!).
kindly always post a new topic instead of posting your query issue on older threads.
can you confirm if you did not make any changes to grade function cell other than replacing None with your codes placed between ###start and end code here###
Also make sure before you submit, click the save emicon on the upper left corner side, before you submit your submission.