Not on Windows right now so I can’t confirm, but you probably forgot to pick “Add Python to PATH” or whatever the option is in the installer. Try running the Python installer again, maybe it’ll let you add it without needing to uninstall & reinstall
Edit: If you’re on Nvidia, there seems to be a simpler install method now: https://github.com/AUTOMATIC1111/stable-diffusion-webui/wiki/Install-and-Run-on-NVidia-GPUs (Method 1)
It’s probably “Add python to environment variables.” They must have changed the wording on that at some point.