NET Core 3.1 - 403 when [Authorize(Roles = "Administrator")]

I'm using Identity Server features, with NET Core 3.1.

What are the requirements in the database when having a role protecting a route?

E.g. [Authorize(Roles = "Administrator")]

1. I have a user created in AspNetUsers table.
2. I have a role created in AspNetRoles table. The value for name is Administrator and NormalizedName is ADMINISTRATOR.
3. I have a row in AspNetUserRoles table with the guid from User and Role.

When hitting this route, I'm getting 403 Forbidden.

Am I missing something?

EDIT 1

The code I'm using add the role is await userManager.AddToRoleAsync(user, "Administrator");

EDIT 2

Here's the Startup.cs file.

using Microsoft.AspNetCore.Authentication;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Identity;
using Microsoft.AspNetCore.Identity.UI;
using Microsoft.AspNetCore.HttpsPolicy;
using Microsoft.AspNetCore.SpaServices.ReactDevelopmentServer;
using Microsoft.EntityFrameworkCore;
using SampleApp.Data;
using SampleApp.Models;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using SampleApp.Entities;
using AutoMapper;
using System;
using SampleApp.Services;
using SampleApp.Middlewares;

namespace SampleApp
{
    public class Startup
    {
        public IConfiguration _configuration { get; }

        public Startup(IConfiguration configuration)
        {
            _configuration = configuration ?? throw new ArgumentNullException(nameof(configuration));

        }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddScoped<ISampleAppRepository, SampleAppRepository>();

            var emailConfig = _configuration
                .GetSection("EmailConfiguration")
                .Get<EmailConfiguration>();

            services.AddSingleton(emailConfig);

            services.AddTransient<IPasswordHasher<User>, PasswordHasher<User>>();

            services.AddScoped<IShippingEmailSender, ShippingEmailSender>();

            services.AddDbContext<ApplicationDbContext>(options =>
            {
                options.UseSqlServer(_configuration["connectionStrings:databaseConnectionString"]);
            });

            services.AddDefaultIdentity<User>(options => options.SignIn.RequireConfirmedAccount = true)
                .AddRoles<IdentityRole>()
                .AddEntityFrameworkStores<ApplicationDbContext>();

            services.AddIdentityServer()
                .AddApiAuthorization<User, ApplicationDbContext>();

            services.AddAuthentication()
                .AddIdentityServerJwt();

            services.AddControllersWithViews(setupAction =>
            {
                setupAction.ReturnHttpNotAcceptable = true;

            }).AddXmlDataContractSerializerFormatters();

            services.AddRazorPages();

            // In production, the React files will be served from this directory
            services.AddSpaStaticFiles(configuration =>
            {
                configuration.RootPath = "ClientApp/build";
            });

            services.AddMvc();

            services.AddAutoMapper(AppDomain.CurrentDomain.GetAssemblies());
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
                app.UseDatabaseErrorPage();
            }
            else
            {
                app.UseExceptionHandler("/Error");
                // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
                app.UseHsts();
            }

            app.UseHttpsRedirection();
            app.UseStaticFiles();
            app.UseSpaStaticFiles();

            app.UseRouting();

            app.UseMiddleware<TenantDetectionMiddleware>();

            app.UseAuthentication();
            app.UseIdentityServer();
            app.UseAuthorization();
            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
                endpoints.MapRazorPages();
            });

            app.UseSpa(spa =>
            {
                spa.Options.SourcePath = "ClientApp";

                if (env.IsDevelopment())
                {
                    spa.UseReactDevelopmentServer(npmScript: "start");
                }
            });
        }
    }
}

I was missing this.

services.AddAuthentication(options =>
{
    options.DefaultAuthenticateScheme = IdentityConstants.ApplicationScheme;
    options.DefaultChallengeScheme = IdentityConstants.ApplicationScheme;
    options.DefaultSignInScheme = IdentityConstants.ExternalScheme;
});

Thanks to this answer! https://stackoverflow.com/a/56473365/779975