How can I specify which Python version poetry should create venv?

Poetry can't fully manage Python versions on its own using the version specified in the pyproject.toml. You can either use a tool like pyenv (or pyenv-win if you're using Windows) for managing multiple versions, or you can use poetry env use path/to/python. If Python 3.9 is in your path or available on your system via pyenv or some similar version management tool, you can use poetry env use python3.9 or poetry env use 3.9. You can read more about poetry env use here.