Cannot resolve scoped service " " from root provider

Question

I'm coding a website using ASP.NET Core 6 from the course. The course was released in winter 2022 with old middleware. So I am facing problems with it. Here I have a DbInitializer file that contains user initialization data.

System.InvalidOperationException: 'Cannot resolve scoped service 'Mango.Services.Identity.Initializer.IDbInitializer' from root provider.'

DbInitializer.cs:

public class DbInitializer : IDbInitializer
{
private readonly ApplicationDbContext _context;
private readonly UserManager<ApplicationUser> \userManager;
private readonly RoleManager<IdentityRole> _roleManager;
public DbInitializer(ApplicationDbContext context, UserManager<ApplicationUser> userManager, RoleManager<IdentityRole> roleManager)
{
_context = context;
_userManager = userManager;
_roleManager = roleManager;
}

        public void Initialize()
        {
            if (_roleManager.FindByNameAsync(SD.Admin).Result == null)
            {
                _roleManager.CreateAsync(new IdentityRole(SD.Admin)).GetAwaiter().GetResult();
                _roleManager.CreateAsync(new IdentityRole(SD.Customer)).GetAwaiter().GetResult();
            }
            else
            {
                return;
            }
    
            ApplicationUser adminUser = new ApplicationUser()
            {
                UserName = "admin@gmail.com",
                Email = "admin@gmail.com",
                EmailConfirmed = true,
                PhoneNumber = "111111111",
                FirstName = "Ben",
                LastName = "Admin"
            };
    
            _userManager.CreateAsync(adminUser,"Admin123").GetAwaiter().GetResult();
            _userManager.AddToRoleAsync(adminUser, SD.Admin).GetAwaiter().GetResult();
    
            var temp1 =_userManager.AddClaimsAsync(adminUser, new Claim[]{
                new Claim(JwtClaimTypes.Name, adminUser.FirstName+" "+ adminUser.LastName),
                new Claim(JwtClaimTypes.GivenName, adminUser.FirstName),
                new Claim(JwtClaimTypes.FamilyName, adminUser.LastName),
                new Claim(JwtClaimTypes.Role, SD.Admin)
            }).Result;
    
            ApplicationUser customerUser = new ApplicationUser()
            {
                UserName = "customer1@gmail.com",
                Email = "customer1@gmail.com",
                EmailConfirmed = true,
                PhoneNumber = "111111111",
                FirstName = "Ben",
                LastName = "Cust"
            };
    
            _userManager.CreateAsync(customerUser, "Admin123").GetAwaiter().GetResult();
            _userManager.AddToRoleAsync(customerUser, SD.Customer).GetAwaiter().GetResult();
    
            var temp2 = _userManager.AddClaimsAsync(customerUser, new Claim[]{
                new Claim(JwtClaimTypes.Name, customerUser.FirstName+" "+ customerUser.LastName),
                new Claim(JwtClaimTypes.GivenName, customerUser.FirstName),
                new Claim(JwtClaimTypes.FamilyName, customerUser.LastName),
                new Claim(JwtClaimTypes.Role, SD.Customer)
            }).Result;
        }
    }

Then my Program.cs file:

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
builder.Services.AddControllersWithViews();

builder.Services.AddDbContext<ApplicationDbContext>(options =>
    options.UseSqlServer(builder.Configuration.GetConnectionString("DefaultConnection")));

builder.Services.AddIdentity<ApplicationUser, IdentityRole>()
    .AddEntityFrameworkStores<ApplicationDbContext>().AddDefaultTokenProviders();

var identityBuilder = builder.Services.AddIdentityServer(options =>
{
    options.Events.RaiseErrorEvents = true;
    options.Events.RaiseInformationEvents = true;
    options.Events.RaiseFailureEvents = true;
    options.Events.RaiseSuccessEvents = true;
    options.EmitStaticAudienceClaim = true;
}).AddInMemoryIdentityResources(SD.IdentityResourses)
.AddInMemoryApiScopes(SD.ApiScopes)
.AddInMemoryClients(SD.Clients)
.AddAspNetIdentity<ApplicationUser>();

builder.Services.AddScoped<IDbInitializer, DbInitializer>();

identityBuilder.AddDeveloperSigningCredential();

builder.Services.AddControllersWithViews();

var app = builder.Build();

// Configure the HTTP request pipeline.
if (!app.Environment.IsDevelopment())
{
    app.UseExceptionHandler("/Home/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.UseRouting();
app.UseIdentityServer();
app.UseAuthorization();
app.Services.GetRequiredService<IDbInitializer>().Initialize();
app.MapControllerRoute(
    name: "default",
    pattern: "{controller=Home}/{action=Index}/{id?}");

app.Run();

I myself have not so long started to dive into the world of asp.net core coding and therefore encounter such problems. Can you tell me how to solve this problem? All the ways I found did not help.