Projects that target Windows 8 cannot be opened because the Tools for Maintaining Store Apps for Windows 8 are not installed. ...

Projects that target Windows 8 cannot be opened because the Tools for Maintaining Store Apps for Windows 8 are not installed. You can either:
1. Retarget your Windows 8 projects to Windows 8.1 to take advantage of new features, or
2. Install the Tools for Maintaining Store Apps for Windows 8 to continue servicing your app.

For more information, see how to retarget your Windows 8 app to Windows 8.1.

Clicking OK will leave your Windows 8 projects unloaded in Visual Studio.