.NET Core 2.1 SDK will be released this week. If you can’t wait until then, add this to your *.csproj
<Project Sdk="Microsoft.NET.Sdk.Web">
<PropertyGroup>
<TargetFramework>netcoreapp2.1</TargetFramework>
<RestoreAdditionalProjectSources>
https://dotnetfeed.blob.core.windows.net/orchestrated-release-2-1/20180515-07/final/index.json
</RestoreAdditionalProjectSources>
</PropertyGroup>
....
</Project>
And download the final SDK from: https://dotnetcli.blob.core.windows.net/dotnet/Sdk/2.1.300/dotnet-sdk-2.1.300-win-x64.exe
For more details visit: https://github.com/aspnet/Home/wiki/2.1.0-Early-Access-Downloads