Postfix exit code 75
Solution 1:
Packages that are "not fully installed or removed" generally must be reinstalled.
sudo apt install --reinstall <package_name>
However, that won't work in your specific case, because postfix exit code 75 usually occurs when you have an invalid hostname.
It's not a dpkg or postfix problem; it's a hostname problem that has been exposed by your package actions.
How to change the hostname depends upon which release of Ubuntu you are running, and whether or not you are willing to reboot the system.