If you are absolutely sure that your Python library path is in your system variables (and you can find that path when you pip install Jupyter, you just have to read a bit) and you still experience “command not found or recognized” errors in Windows, you can try:
python -m notebook
For my Windows at least (Windows 10 Pro), having the python -m is the only way I can run my Python packages from command line without running into some sort of error
Fatal error in launcher: Unable to create process using ' "
or
Errno 'THIS_PROGRAM' not found