popup message box code runs in Windows, but not in Linux

[wavic]

It's obvious that you broke something in your system. The Kubuntu is Ubuntu system with KDE plasma desktop installed and perhaps some additional features so one can call it distro. If Ubuntu, Lubuntu, Xubuntu, Mint, Ubuntu-MATE work, Kubuntu should work too without glitches.

Try:

sudo apt update
sudo apt install -f
sudo apt upgrade

Restart the system

sudo apt autoremove
sudo apt autoclean

After all of that reinstall Python.

sudo apt install --reinstall python python3

Try to run the script again.