I have already installed VS2017 Enterprise 15.6.4
. Recently installed .NET Core 2.1.101 SDK
from official site
I was expecting .NET Core
I have faced the same problem. I solved this by installing the right SDKs with Runtime for .NET Core 2.1.
Basically to run/create 2.1 projects from VS Preview, you need to install the "2.1.300" (not 2.1.4) .NET Core SDKs and Runtime. https://www.microsoft.com/net/download/dotnet-core/sdk-2.1.300-preview1 (this includes required Runtime already)
The good explanation of this misleading with versions i found here https://github.com/dotnet/cli/issues/8309 (dasMulli commented on Feb 24).