If Visual Studio 2017 is not displaying the relevant Target Frameworks for your machine then it may be that the targeting packs are not installed on your machine.

Run the Visual Studio Installer (from within Visual Studio, select Tools | Get Tools And Features) and select the necessary targeting packs.

When you restart Visual Studio you will find the Target Frameworks have been added to the list.

Useful links…

Troubleshoot blocked .NET Framework installations and uninstallations

How to: Determine Which .NET Framework Versions Are Installed