Solution
Python installed within virtual environments cannot directly communicate with LabVIEW using the Python Node with the latest version of LabVIEW (2021). This is due to the way the environment wraps the Python interpreter, making LabVIEW unable to find the interpreter installation and unable to make the necessary calls.
The officially supported Python environments that can communicate with LabVIEW 2018 up to 2020 are native Python 2.7 and 3.6 installed from The Python Software Foundation. LabVIEW 2021 added support for Python 3.7, 3.8, and 3.9. It maintains support for Python 3.6 but deprecates support for Python 2.7.