It immediatly closes after running

[DHMOLS]

The version you just installed should uninstalled:
python -m pip uninstall PySimpleGUI
python -m pip cache purge

Then install the latest from the private server:
python -m pip install --upgrade --extra-index-url https://PySimpleGUI.net/install PySimpleGUI

You can also force a reinstall using this command and it'll install the latest regardless of what you have installed currently
python -m pip install --force-reinstall --extra-index-url https://PySimpleGUI.net/install PySimpleGUI

Use python3 command if you're running on the Mac or Linux

[JuergenFleiss]

Same for me; seems the problem is that PySimpleGUI removed all their open source packages;

For the command
python -m pip install --upgrade --extra-index-url https://pysimplegui.net/install PySimpleGUI

https://pysimplegui.net/install seems gone too now, they only seem have the commercial paid packages now...

I opened a pull request replacing the package with one of the open source copies of the old PySimpleGUI #10

Until the pull request is reviewed and merged by @preshtildeath you can use the fork: https://github.com/JuergenFleiss/print-proxy-prep