All test pass but grade error

That error typically means that there is something corrupted or damaged about your notebook at the “metadata” level. Meaning that it’s not that your code is wrong, but that something has damaged the other structural parts of the notebook. Did you edit the notebook on any other platform besides the course website?

My suggestion as the next step would be to get a fresh copy of the notebook by following this procedure. As part of that process, you have to rename your existing notebook. Once you have the clean copy, then very carefully “copy/paste” over just your completed code segments from the “YOUR CODE HERE” sections of the original notebook and try the grader again.