How to reference ASP.NET Core 6 types in .NET 6 library?

You need to add a FrameworkReference to your project file instead of a PackageReference as described here.

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

Right-click your project in the Solution Explorer and select “Edit Project File” to open the file and look for the <ItemGroup> section, creating a new one if it does not exist.