Visit the Discussion forum to see if your peers are experiencing or have found resolutions for similar errors. If the error isn’t resolved in 24 hours, please reach out to Coursera through our Help Center.
I have to be a little careful here, because I have not taken this course, but I have seen that error message from other graders. It generally means that something is damaged about your notebook, independent of whether your code is correct or not. The notebook is a JSON file and there are lots of structures there that the grader uses which are not directly visible to us.
My suggestion would be to get a new clean copy of the notebook and then very carefully “copy/paste” over just your completed code segments and see if that helps. There is a procedure for how to get a clean copy of a notebook on the DLS FAQ Thread. See the first topic. I’m pretty sure those instructions are generic enough that they should work in NLP as well. If you have trouble following that link, let me know.