Sudipta DM’ed me the notebook. The return statement has simply been deleted from the model function, which is why that error occurs. There should have been no need to modify that code at all, so presumably this was some sort of accident. The best thing to do would be to get a clean copy of the notebook and then “copy/paste” your code from only the “YOUR CODE HERE” sections over to the clean copy, in case there is other similar damage to the parts that didn’t need to be changed.