The problem is MSFS 2024 when you change the Content folder with the MyLibrary settings, won’t create an actual Community folder until you start it again (unless your Community folder already existed), so the FSDT Installer won’t find it, because it needs the folder to be there, to prevent being fooled by remnants of old installations.
So you need to either create an empty Community folder yourself, or start the sim again, so it will create it on the new startup, and finally the FSDT installer.