This will install a number of extensions to \\Anaconda3\\lib\\site-packages\\jupyter_contrib_nbextensions\\nbextensions\\
There are pip install options in the guide, but I haven't tried this for a manual installation of Jupyter Lab and Jupyter Notebook
Run jupyter contrib nbextension install --user
From the Anaconda Prompt, run jupyter notebook
During loading of the Jupyter Notebook environment, there where warnings as follows [jupyter_nbextensions_configurator] nbextension '...' has duplicate listings.
One location was \\Anaconda3\\..., as mentioned above, and the other was \\Users\\...\\AppData\\\Roaming\\jupyter\\nbextensions\\...
This could be because I had a previous installation of Anaconda, but I resolved this issue by renaming \\Users\\...\\AppData\\\Roaming\\jupyter\\nbextensions\\... to \\Users\\...\\AppData\\\Roaming\\jupyter\\nbextensions_old\\...
Go to the Nvextensions tab, select the option for Table of Contents (2), and Enable it.
Quit / Logout of Jupyter Notebook
From the Anaconda prompt, go the jupyter lab
Now all of the Export Notebook As... options will be enabled in the file menu.
Including, and specifically Export Notebook to Html_toc
Examples
Here are examples of HTML notebooks with a Table of Contents output from Jupyter Lab. See the DataCamp notebooks.