I’m not a mentor for NLP, but this sounds to me like the “grader metadata” in your notebook is somehow corrupted. My suggestion is to get a clean copy of the notebook and then very carefully go through and just “copy/paste” over only your entered code from the “YOUR CODE HERE” blocks from your current notebook. Then try submitting again and see if that helps.
The first topic on the DLS FAQ Thread gives instructions for how to do that process. The instructions are generic and apply to NLP or MLS as well. Please have a look and give that a try and let us know if it helps or not.