W2_A2_Submission error in Jupyter Notebook

That is because you renamed the notebook. The “Work in Browser” link only opens the standard name of the notebook. Also note that clicking “Submit” in any copy of the notebook will also only grade the official copy (the same one opened by “Work in Browser”).

So generally speaking it is a mistake to just rename the notebook without first making a copy.

You can get back the original notebook by following the “Get a Clean Copy” procedure on the DLS FAQ Thread. Once you have the clean copy, open that and then rename it to a different name and rename your previous copy back to the standard name. From the clean copy, first do “File → Rename” and then do “File → Open” and find your previous version and then rename that.