IHttpClientFactory in .NET Core 2.1 Console App references System.Net.Http

Add this package reference in your csproj.

<PackageReference Include="Microsoft.AspNetCore.App" Version="2.1.2" />

The Microsoft.Extensions.Http, which is default included in the Microsoft.AspNetCore.App package, contains lots of packages which is commonly used for http-related code, it includes the System.Net package for example.

When you use something from the nested packages of Microsoft.Extensions.Http, you still need to reference them by the using statement.

So, nothing is wrong here. Just add a the using System.Net.Http; to your class.

The official documenation suggests that all I need to add to my project is a reference to the Microsoft.Extensions.Http NuGet package. I've done this.

That's true but in order to make things easier, you have to add Microsoft.Extensions.DependencyInjection as a NuGet package, in fact, you can delegate all the creation of httpClient instance to the HttpClientBuilderExtensions which add a lot of extensions methods to create a named or typed HTTPClient here I have written an example for you

using System;
using System.Net.Http;
using System.Threading.Tasks;
using Microsoft.Extensions.DependencyInjection;


namespace TypedHttpClientConsoleApplication
{
    class Program
    {
        public static void Main(string[] args) => Run().GetAwaiter().GetResult();
        public static async Task Run()
        {
            var serviceCollection = new ServiceCollection();


            Configure(serviceCollection);

            var services = serviceCollection.BuildServiceProvider();

            Console.WriteLine("Creating a client...");
            var github = services.GetRequiredService<GitHubClient>();

            Console.WriteLine("Sending a request...");
            var response = await github.GetJson();

            var data = await response.Content.ReadAsStringAsync(); 
            Console.WriteLine("Response data:");
            Console.WriteLine((object)data);

            Console.WriteLine("Press the ANY key to exit...");
            Console.ReadKey();
        }
        public static void Configure(IServiceCollection services)
        {







            services.AddHttpClient("github", c =>
            {
                c.BaseAddress = new Uri("https://api.github.com/");

                c.DefaultRequestHeaders.Add("Accept", "application/vnd.github.v3+json"); // GitHub API versioning
                c.DefaultRequestHeaders.Add("User-Agent", "HttpClientFactory-Sample"); // GitHub requires a user-agent
            })                        
            .AddTypedClient<GitHubClient>();
        }
        private class GitHubClient
        {
            public GitHubClient(HttpClient httpClient)
            {
                HttpClient = httpClient;
            }

            public HttpClient HttpClient { get; }

            // Gets the list of services on github.
            public async Task<HttpResponseMessage> GetJson()
            {
                var request = new HttpRequestMessage(HttpMethod.Get, "/");

                var response = await HttpClient.SendAsync(request).ConfigureAwait(false);
                response.EnsureSuccessStatusCode();

                return response;
            }
        }

    }

}

Hope this help