error CS7069: Reference to type 'AuthenticationOptions' claims it is defined in 'Microsoft.AspNetCore.Authentication', but it could not be found

I'm using Asp.net Core 2 and trying to add OAuth2 support, I'm following this example here: https://www.codeproject.com/Articles/1185880/ASP-NET-Core-WebAPI-secured-using-OAuth-Client-Cre

Unfortunately on the line: 

app.UseIdentityServerAuthentication(new IdentityServerAuthenticationOptions            {                Authority = "http://localhost:50151",                RequireHttpsMetadata = false,                ApiName = "scope.readaccess"            });

I get the error seen in the title of this post. I'm confused by this, from googling it looks like this feature was supposed to be added for asp.net core 2 release but it doesn't look like it has?