The Addon Manager doesn't have anything to do with this, of course. And those messages you see are NOT coming from it.
As you can see in your first screenshot, the message window that gave you the error is titled "Simconnect Client", so the error is caused by it, not the Addon Manager. The Addon Manager installer simply launches the MS Simconnect installer you already had in your flight sim SDK folder, which is required to run addons under FSX Steam.
The "Account already exists" indicates you have a problem with your registry keys, so Windows .MSI installers don't work correctly.
See here if this can help:
https://support.microsoft.com/it-it/mats/program_install_and_uninstall/en