Namespace issues in VS2017 and Win7 with Xamarin app?

问题

I have VS2017 Community edition installed in a Windows 7 PC and a Windows 10 laptop. They both have the exact same installation.

Last night I created a Cross-Platform > Mobile App (Xamarin.Forms) project in my Win10 laptop, and it ran perfectly in the android emulator. No issues there.

I did the same exact thing in my Windows 7 PC, and I'm getting tons of compiler errors. All of them are of the The type or namespace name 'System' could not be found nature.

Is there anything I need to do in Windows 7 so these projects run?

In both environments (Win7 and Win10) I did the exact same thing: installed VS2017, created the new Xamarin cross-platform project, and debugged it. In Win10 it runs flawlessly while in Windows 7 I get a bunch of compiler errors with System namespace, which is literally the most fundamental namespace of the .NET Framework.

Any help is appreciated.

I'd like to add that I created a regular ASP.NET Web Application in Windows 7 and it runs flawlessly (no issues with System namespace), so it's an error with Xamarin cross-platform.