c++ – Python Package installed with pip could not be found (Windows)

I have been given two large undocumented C++ and Python projects and I want to understand their structures. For this I would like to create class diagrams with the Python package Sphinx. I opened the command line on Windows 10, installed Python 3.9.6 from the Windows Store and then I typed

pip install sphinx

to install Sphinx globally. When I rerun the command, it tells me that Sphinx is already installed. When I run

sphinx-quickstart

an error is displayed telling me I mistyped the command or that it could not be found. I think Windows does not know the path to Sphinx and I do not know how to fix the problem. I sticked to the official installation guide.