From time to time we do retire older versions of Skype to focus our efforts on improving the most recent versions of Skype. When this happens, you'll see a message like this when trying to sign in to Skype:
Simply click Download the latest version now and follow the steps to upgrade to the latest version for free. You can also follow detailed step-by-step instructions here to resolve the issue and continue using Skype.
Skype currently supports Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Vista, and Windows XP.