Unable to install pl/python for Postgresql12
Using set at the command prompt does not give feedback upon success.  To verify it was actually set, you would run echo %PYTHON_HOME%.  But it looks like you have a space after c:\ that should not be there.
Also, this only sets the variables for the particular cmd session you run it in, so it works only if you are starting PostgreSQL from that command prompt.  That isn't how you usually run PostgreSQL on Windows, you normally run it as a service.  So you have would have to use the GUI interface to &quot;edit the system environment variables&quot;, not use set from the command line.