I'm trying to set pythonpath but when I follow in some example in other stackoverflow, I just get a red message or nothing happened.
In my picture, u can see, no my folder D:\DemoPython
Does anyone have exactly how to set it?, please give me some examples or the exact answer would be better. Thank so much.
The syntax for setting environment variables you found is for cmd.exe (the DOS-like prompt that has shipped with Windows for ages). You're running in PowerShell, which is newer, and has significantly different syntax.
On PowerShell (which you're using), you want:
$env:PYTHONPATH = "D:\DemoPath;${env:PYTHONPATH}"
or
$env:PYTHONPATH = "${env:PYTHONPATH};D:\DemoPath"
depending on whether you want to take precedence over existing entries or not, respectively.
My Computer > Properties > Advanced System Settings > Environment Variables >
Just add the path as :
C:\Users\zee\AppData\Local\Programs\Python\Python39 (or wherever you installed python)
PYTHONPATH persistently is usually a bad idea (if you want something installed persistently, make it actually installable with setup.py/setup.cfg or the like and install it; you won't need customized environment variables); the code it references must work (or at least be syntactically valid) in all versions of Python that imports it, which can be a problem (it's a huge problem when you're still mixing Py 2 and Py 3).
cmd.exe(DOS-like) prompt, but using PowerShell. You need to match the syntax to the shell.