It appears from the grader feedback error that you may not be working with the latest version of the assignment.
Please follow the instructions in the attached post to reset your assignment. Be sure to either delete or rename your current notebook before using Get Latest Version to obtain a fresh copy.
After updating the assignment, solve the exercises again, run all the cells, and submit the assignment. If you are still experiencing the issue, please share the grader feedback with us again so we can take a closer look. If the problem persists, we can also notify the course staff for further investigation.