Hello,
I have found a interesting bug with python 3.9 and building WebGL projects in Unity (2020.3.xx). The problem I had was during the IL2cpp compiling portion of the build-out and it would throw an error in C:*PATH_TO_PYTHON_3.9 \setup.py* I do not have the error logs anymore but every build would stop at the “IL2cpp” portion.
The fix I had to do was take off the %PYTHONHOME% system environment variable and the build out would succeed. Also note I have tried this with previous versions of Unity and with empty projects so it could not be possible for plug ins to interfere. Is this a known issue in the Unity Editor? Surely there has to be another fix then deleting environment variables.