Duplicate of BUG-000138231: This is an issue with ArcGIS API for Python documented in BUG-000138231 and fixed in version 2.0.0.
ArcGIS Notebook Server 10.8.1 ships with ArcGIS API for Python 1.8.1.
If users want to use ArcGIS API for Python 1.8.1, create a custom notebook runtime that has ArcGIS API for Python 2.0.0.
Extend a notebook runtime: https://enterprise.arcgis.com/en/notebook/10.8/install/windows/extend-a-notebook-runtime.htm.