محتوای فایل startup رو قرار بدید.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Identity;
using Microsoft.EntityFrameworkCore;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using DrBND.Data;
using DrBND.Models;
using DrBND.Services;
using Microsoft.Extensions.FileProviders;
using System.IO;

namespace DrBND
{
    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.AddDbContext<ApplicationDbContext>(options =>
                options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));

            services.AddIdentity<ApplicationUser, IdentityRole>()
                .AddEntityFrameworkStores<ApplicationDbContext>()
                .AddDefaultTokenProviders();
            //------------------------------------------

            //----------------------------------تنظیمات رول------------------------
            //Password Strength Setting 
            services.Configure<IdentityOptions>(options =>
            {
                // Password settings 
                options.Password.RequireDigit = true;
                options.Password.RequiredLength = 6;
                options.Password.RequireNonAlphanumeric = false;
                options.Password.RequireUppercase = false;
                options.Password.RequireLowercase = false;
                options.Password.RequiredUniqueChars = 0;

                // Lockout settings 
                options.Lockout.DefaultLockoutTimeSpan = TimeSpan.FromMinutes(60);
                options.Lockout.MaxFailedAccessAttempts = 10;
                options.Lockout.AllowedForNewUsers = true;

                // User settings 
                options.User.RequireUniqueEmail = false;
            });

            //Setting the Account Login page 
            services.ConfigureApplicationCookie(options =>
            {
                // Cookie settings 
                options.Cookie.HttpOnly = true;
                options.ExpireTimeSpan = TimeSpan.FromMinutes(30);
                options.LoginPath = "/Home/Login"; // If the LoginPath is not set here,  
                                                      // ASP.NET Core will default to /Account/Login 
                options.LogoutPath = "/Home/Logout"; // If the LogoutPath is not set here,  
                                                        // ASP.NET Core will default to /Account/Logout 
                options.AccessDeniedPath = "/Home/AccessDenied"; // If the AccessDeniedPath is  
                                                                    // not set here, ASP.NET Core  
                                                                    // will default to  
                                                                    // /Account/AccessDenied 
                options.SlidingExpiration = false;
            });



            //------------------------------------------------------------------------------


            // Add application services.
            services.AddTransient<IEmailSender, EmailSender>();

            services.AddMvc();
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IHostingEnvironment env, IServiceProvider services)
        {
            if (env.IsDevelopment())
            {
                app.UseBrowserLink();
                app.UseDeveloperExceptionPage();
                app.UseDatabaseErrorPage();

            }
            else
            {
                app.UseExceptionHandler("/Home/Error");
            }

            app.UseStaticFiles();

            app.UseAuthentication();

            app.UseMvc(routes =>
            {
                routes.MapRoute(
                    name: "default",
                    template: "{controller=Home}/{action=Index}/{id?}");
            });
            //----------------------
            //CreateUserRoles(services).Wait();
        }

        //private async Task CreateUserRoles(IServiceProvider serviceProvider)
        //{
        //    var RoleManager = serviceProvider.GetRequiredService<RoleManager<IdentityRole>>();
        //    var UserManager = serviceProvider.GetRequiredService<UserManager<ApplicationUser>>();

        //    IdentityResult roleResult;
        //    //Adding Admin Role 
        //    var roleCheck = await RoleManager.RoleExistsAsync("Admin");
        //    if (!roleCheck)
        //    {
        //        //create the roles and seed them to the database 
        //        roleResult = await RoleManager.CreateAsync(new IdentityRole("Admin"));
        //    }
        //    //Assign Admin role to the main User here we have given our newly registered  
        //    //login id for Admin management 
        //    ApplicationUser user = await UserManager.FindByEmailAsync("syedshanumcain@gmail.com");
        //    var User = new ApplicationUser();
        //    await UserManager.AddToRoleAsync(user, "Admin");
        //}
    }
}

در کلاس Startup همه چی درست به نظر میرسه. لطفا Web.config رو به طور کامل قرار بدید.

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <location path="." inheritInChildApplications="false">
    <system.webServer>
      <handlers>
        <add name="aspNetCore" path="*" verb="*" modules="AspNetCoreModule" resourceType="Unspecified" />
      </handlers>
      <aspNetCore processPath="dotnet" arguments=".\DrBND.dll" stdoutLogEnabled="false" stdoutLogFile=".\logs\stdout" />
		<environmentVariables>
     <environmentVariable name="ASPNETCORE_ENVIRONMENT" value="Development" />
 </environmentVariables>

    </system.webServer>
  </location>
</configuration>

Environment را داخل aspNetCore قرار نداده بودید:

   <aspNetCore processPath="dotnet" arguments=".\DrBND.dll" stdoutLogEnabled="false" stdoutLogFile=".\logs\stdout">
        <environmentVariables>
          <environmentVariable name="ASPNETCORE_ENVIRONMENT" value="Development" />
        </environmentVariables>
      </aspNetCore>