I feel your frustration. These example Notebooks would be a great resource, if they were kept up-to-date, so those with little Python experience could learn from them. As it is, these Notebooks work briefly when they first come out, and then most stop working completely due to updates in the ArcGIS API for Python and/or the ArcGIS Online Notebook environment. Those updates usually trigger issues that someone new to Python likely does not have the right skills yet to diagnose, so they waste time and get nowhere. It would be nice if Esri committed to keeping the examples up-to-date.
As of a few weeks ago, the missing step to getting SignImageClassificationModel working was to ensure you ran it using the correct, older runtime in an ArcGIS Online Notebook. So whether you download and then upload the Notebook, used Save As, or are cutting-and-pasting into your own Notebook, go to your Notebook's Settings, and change the runtime to "ArcGIS Notebook Python 3 Advanced with GPU support - 4.0". (See Specify the runtime of a notebook.)
Once you've done that, then following the steps in the Notebook exactly -- installing version 1.8.4 of the API, restarting the kernel afterwards -- should result in a fully working Notebook.
