System.AggregateException:

Below is the error that I am getting in program cs. I am using aspnet core mvc 3.1

 Some services are not able to be constructed (Error while validating the service descriptor 'ServiceType: Microsoft.AspNetCore.Identity.RoleManager`1[Microsoft.AspNetCore.Identity.IdentityRole] Lifetime: Scoped ImplementationType: 

Here is the program:

using Microsoft.AspNetCore;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Hosting;

namespace Bumples15
{
public class Program
{
public static void Main(string[] args)
{
CreateHostBuilder(args).Build().Run();  //here is where they say the problem is.
}

public static IHostBuilder CreateHostBuilder(string[] args) =>
Host.CreateDefaultBuilder(args)
.ConfigureWebHostDefaults(webBuilder =>
{
webBuilder.UseStartup<Startup>();
});

}
}

Below is the startup page

using Bumples15.Models;
using Bumples15.Service;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Identity;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using System;

namespace Bumples15
{
public class Startup
{
public Startup(IConfiguration configuration)
{
Configuration = configuration;
}

public IConfiguration Configuration { get; }

// This method gets called by the runtime. Use this method to add services to the container.
public void ConfigureServices(IServiceCollection services)
{

services.AddControllersWithViews();

services.AddDbContext<ApplicationDbContext>(options =>
options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
services.AddDefaultIdentity<IdentityUser>()
.AddEntityFrameworkStores<ApplicationDbContext>();

services.AddScoped<RoleManager<IdentityRole>>();
services.AddScoped<IRepository<Product>, SqlBooksRepository>();
services.AddScoped<IRepository<Order>, SqlOrdersRepository>();

services.AddRazorPages();
}

// 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.UseHttpsRedirection();
app.UseStaticFiles();

app.UseRouting();

app.UseAuthentication();

app.UseEndpoints(endpoints =>
{
endpoints.MapControllerRoute(
name: "default",
pattern: "{controller=Home}/{action=Index}/{id?}");
endpoints.MapRazorPages();
});
}
}
}

The course that I am using is a 2.1 program but I have tried to update it to 3.1. Where am I going wrong,

Thanks,

Jen