Yes, thank you for following the guidelines and not posting your code. Also please note that the mentors can’t directly view your notebook.
That type of error from the grader usually means something structural is wrong with your notebook. Did you add any cells or work on the notebook on some other platform and then re-upload it to Coursera? If so, that is very likely to cause issues like this. In that case one thing to try is to get a clean copy of the notebook and carefully “copy/paste” over your code only in the “YOUR CODE HERE” sections. Here are the instructions for how to get a clean copy.