After installing with pip, "jupyter: command not found"

Try

python -m notebook

Or, if you used pip3 to install the notebook:

python3 -m notebook

On Mac OS Catalina and brewed Python3.7

To be able to run jupyter notebook from terminal, you need to make sure that ~/.local/bin is in your path.

Do this by running export PATH=$PATH:~/.local/bin for your current session, or adding that line to the end of ~/.bashrc to make your changes last for future sessions (e.g. by using nano ~/.bashrc). If you edit ~/.bashrc you will need to log out and log back in to make see your changes take effect.

you did not log out and log in ? It should be on your path to execute. If not, pip installed executables in .local, so in a terminal:

 ~/.local/bin/jupyter-notebook

should start notebook