If you installed newly released version of Visual Studio 2017 and started a fresh ASP.NET Core project targeting .NET Core, you might encounter the below error message.
The type or namespace name ‘System’ could not be found (are you missing a using directive or an assembly reference?)
Also, looks like there are problems with NuGet packages as in below screenshot
This is very strange. Looks like the problem was due to NuGet package source pointing to a folder under Visual Studio 2015 (previous version) folder.
Go to Tools | NuGet Package Manager | Package Manager Settings – checked ‘Allow NuGet to download missing packages’ and ‘Automatically check for missing packages during build in VS’ and then clicked ‘Clear All NuGet Cache(s)’. Then re-built the solution.