HttpClient.cs Authentication Error?

Program.cs

using System;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleTestApp1
{
    class Program
    {
        public static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");
            SeedDataInline2().GetAwaiter().GetResult();
            Console.ReadLine();
        }

        public static async Task SeedDataInline2()
        {
            // Arrange
            var client = new HttpClient();

            //client.DefaultRequestHeaders.Clear();
            //client.BaseAddress = new Uri(settings.Url);

            var login = "abc";
            var password = "def";
            var requestUri = "https://blah/blah/blah/1";

            var credentials = Convert.ToBase64String(Encoding.ASCII.GetBytes($"{login}:{password}"));

            //client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", credentials);
            //client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

            var request = new HttpRequestMessage(HttpMethod.Get, requestUri);

            request.Version = Version.Parse("1.1");
            request.Headers.Authorization = new AuthenticationHeaderValue("Basic", credentials);
            request.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

            //ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;

            // Act     
            var response = await client.SendAsync(request);

            // Assert     
            Console.WriteLine(response.StatusCode);
            response.EnsureSuccessStatusCode();
        }
    }
}