When trying to install an application, you can get this error even if another installation isn’t running:
Another installation is already in progress. Complete that installation before proceeding with this install.
This is typically because an installation started but didn’t temrinate properly, and / or an installation is happening in the background.
To fix this, cancel your install then go to the Task Manager (Ctrl+Shift+Esc) and terminate all MSIEXEC proccesses that are running. You should now be able to install your application without Windows complaining that another installation is already in progress.