It seems there is suddenly something out of sync with asp.net core packages.
It looks like Microsoft.AspNetCore.App 2.1.1 was released 16 hours ago on nuget.org but
The 2.1.1 (preview) SDK is available on their GitHub.
You can download directly here:
https://dotnetfeed.blob.core.windows.net/orchestrated-release-2-1/20180605-09/final/assets/Sdk/2.1.301-preview-008906/dotnet-sdk-2.1.301-win-x64.exe
See:
https://github.com/dotnet/versions/tree/7a833ffffdfddc27f2074b755b94234a25b9757637/build-info/dotnet/product/cli/release/2.1
We are still waiting on the official SDK...
Edit:
If you are having trouble building, add the following to your .csproj
true
Update:
Official 2.1.1 SDK now released: https://www.microsoft.com/net/download/thank-you/dotnet-sdk-2.1.301-windows-x64-installer