If you are confident of your codes, then I want you to do the below steps.
Get a fresh copy of the assignment by following the below link
Now once you have got a fresh copy, make sure write codes only between ###START and END CODE HERE### especially only replacing None between these markers.
Do not edit any other part of cells, or remove any part of header which mentions Grade 1 or mentions grade cell headings.
Do not add any codes in unit test cells.
You re-write the codes referring to your old copy but make sure only mentions codes where it is instructed to do so.
Let me know if this give you a different result.
Regards
DP