Please, before you submit your notebook a very good thing to do is to restart your notebook and run it all again to check that everything is working properly.
Based on the error message, you have changed code outside of the exercise cell, please reset your notebook by following these steps which will ask you to rename your current notebook so that a new version can be copied to your workspace. Then you can move your exercise code from your current notebook to the new one, and submit again.
Please be reminded not to change anything outside these sections: