This could be due to any editing done outside of required/mentioned are where you weren’t suppose to edit/add/remove any part of code from the assignment.
The best way to resolve this issue would be to save a copy, then get a fresh copy and mention or write codes only between ###START AND END CODE HERE### and then re-submit.
To get a fresh copy refer the below link(please make sure you have saved a copy of your already done assignment, so you can compare once you are re-doing the successful submission of your assignment.
Let me know if the issue persisted!!!
Regards
DP