If you're trying to install products included in the subscription plan, please check mExtension
If you're trying to install a product purchased under a perpetual license, you are most likely using an outdated version of mInstaller.
Please close the app (either via CMD+Q or using the Activity Monitor), delete it from your Applications folder (be sure to also delete all other existing copies), empty your trash(!), and download the latest version
Do not launch it yet though and instead, go to your Applications folder, press and hold the CMD+Option(Alt) keys, double-click mInstaller.app, and when asked, delete the app's preferences.
Finally, try logging in to mInstaller and the problem should be solved.