Programming assignment won't grade

It looks like the “metadata” in your notebook has gotten damaged or corrupted somehow. The notebooks are JSON text files that contain some information that is not visible in the UI, but which is used by the grader infrastructure. My suggestion would be to get a clean copy of the notebook by using these instructions (that is from DLS, but the notebook mechanisms are the same in all the courses). Then very carefully “copy/paste” over just your completed code from the “YOUR CODE HERE” sections from your damaged notebook to the clean copy. Then try submitting the new copy with the added code. Please let us know whether that helps or not.