Only the course staff can look directly at your notebooks and they are (usually) too busy to be listening here. That message most likely means that something is corrupted about your notebook. Did you download it and then edit it with some external tool or in some other way modify it outside the context of the course website? One thing to try is to use the “Get a Clean Copy” procedure documented on the DLS FAQ Thread (also works for the GANs notebooks, since it just uses generic mechanisms) and then carefully “copy/paste” over just your completed code from the “YOUR CODE HERE” segments to the new clean notebook. Then submit again and see if that helps.
Note that some of the other topics on the DLS FAQ Thread are also applicable here: e.g. if you have made a copy of your notebook and are working in a notebook other than the one that is opened by the “Work in Browser” link, you probably don’t have a “Submit Assignment” button in the copy. But if by chance you do, clicking “Submit Assignment” does not grade the renamed copy even if that’s where you clicked the button: it always grades the “official” copy (the one opened by the “Work in Browser” link). Yes, I agree, that’s a violation of the PLA (Principle of Least Astonishment 
), but that’s how it works.