The error message is telling you to add Python 3 to your path.
To do that, use a text editor to open /Users/<you>/.profile, and as the very last line add:
export PATH=/Users/<you>/Library/Python/3.8/bin:$PATH
Then you’ll need to launch a new Terminal window for those settings to take effect. (you could make it take effect in your current shell by entering the line directly into your shell)
[Edit: now that macOS 12.3 has removed all versions of Python, Homebrew is the easiest way to install Python. Fortunately things are simpler because there’s only one version of Python on your system and you won’t need to override the system’s version (because there isn’t one any longer).]