The specified framework 'Microsoft.AspNetCore.App', version '2.1.0' was not found

.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

By updating all NuGet packages, you seem to have upgraded from the RC version you started out with (which is installed on your system) to the RTM version currently being released (and thus already available on NuGet but not installed on your system).

The Microsoft.AspNetCore.App NuGet packages are supposed to be referenced without a version so that the installed tooling can pick an appropriate version.

Update your .csproj file to remove the Version attribute and only reference that package through:

<ItemGroup>
  <PackageReference Include="Microsoft.AspNetCore.App" />
</ItemGroup>

.NET Core SDK 2.1.300 is now available: https://www.microsoft.com/net/download/windows Install it, this fixed the problem for me.