To create or open applications targeting .NET Core 2.1, Visual Studio 2017 15.7 or newer is recommended. Right now 15.7 is only available as a preview, so please use the latest Visual Studio 2017 Preview.
Visual Studio 2017 15.6 supports creating and opening applications targeting .NET Core 2.1, but it contains known issues so 15.7 or newer is recommended.
Visual Studio 2017 15.5 does not support creating or opening applications targeting .NET Core 2.1.