That type of error usually means there is something structural that is wrong with your notebook. But there is one easy experiment to try first:
Kernel -> Restart and Clear Output
Save
Submit again
Please try that and let us know if it helps. The point is that the grader does not need to see your output and sometimes the output can contain syntax that confuses the grader.
If it still fails in the same way after that, then I suggest you start with a fresh copy. Rename your current notebook to save your work and then do the “Get a Clean Copy” procedure. Then carefully “copy/paste” just your code from the “YOUR CODE HERE” segments over to the clean copy of the notebook and submit that.
Let us know if that helps.