@Brian Smith Thanks, Please use http://ml.azure.com, A great notebook experience with productivity features, high reliability, and great performance! Please follow the doc new and improved Studio Notebooks experience.
On January 15, 2021 the Azure Notebooks public preview site retired and has been replaced with integrated services from Visual Studio, Azure, and GitHub. Azure ML released new integrated notebook editor for general availability. This new editor into the Azure ML Studio UI to maximize discoverability, productivity, sharing and collaboration with Jupyter notebooks, a key tool for data scientists.