Disappointed I can't use Duolingo Japanese on my Windows Based system
What version of Windows do you have? (IE: Windows XP, Windows Vista, Windows 7, Windows 8, Windows 8.1, Windows 10. There are other versions before that but I doubt you have any of those.)
If you mean the app instead of the website, you can use it through BlueStacks.
It happens if the support for your OS has stopped Win XP, Win 2000 or anything older. Uninstalled components would not be available for download. In that case you need to replace your OS or the whole computer.
You can also check whether you are using an account without administration rights, which you would need for installing new components.