This is a situation that occurs when a user is using a version of the ArcGIS API for Python lower than version 1.8.3. As the new version of the ArcGIS Workforce Python module that supports offline-enabled projects was released only in early August 2020, user must install that version separately, which can be done using Anaconda:
conda install -c esri/label/prerelease -c esri arcgis==1.8.3
Including this line in the top cell of a hosted Notebook allows the kernel to successfully resolve ArcGIS API for Python 1.8.3 for Workforce notebooks. Refer to https://github.com/Esri/workforce-scripts/blob/master/README.md for more information.