"Grading failed" when submitting the assignment

Don’t worry about the deadlines: they are meaningless in the sense that there is no penalty for missing them. This is documented under “Course Info” on the left menu panel.

Can you show us the actual error you get from the grader? Under the link “Show grader output” on the “My Submission” tab.

Generally speaking, this probably means that there is some portion of the notebook that the grader uses that has been accidentally damaged or corrupted. One thing to try is to get a clean copy of the notebook and then carefully “copy/paste” over just your completed code in the “YOUR CODE HERE” segments. Here’s a thread about how to get a clean copy.