Why is my website working only if I compile while the index file is selected in visual studio?

Question

So I was practicing on how to make a website using asp.net . I was following a course for full stack web development on udemy.

I made multiple c# classes that I still dont know the use for right now(i didnt get to that point) like BLL, DAL, DTO and UI.

Now the UI is where Im working on right now. And thats where I have my index file. I have CSS done JS done and its working well. My Area admin controller seems to be working as well

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using DTO;
namespace UI1.Areas.Admin.Controllers
{
    public class LoginController : Controller
    {
        // GET: Admin/Login
        public ActionResult Index()
        {
            UserDTO dto = new UserDTO();
            return View(dto);
        }
        [HttpPost]
        public ActionResult Index(UserDTO dto)
        {
            return View(dto);
        }
    }
}

And the admin are registration seems okay.

using System.Web.Mvc;

namespace UI1.Areas.Admin
{
    public class AdminAreaRegistration : AreaRegistration 
    {
        public override string AreaName 
        {
            get 
            {
                return "Admin";
            }
        }

        public override void RegisterArea(AreaRegistrationContext context) 
        {
            context.MapRoute(
                "Admin_default",
                "Admin/{controller}/{action}/{id}",
                new {action = "Index", id = UrlParameter.Optional }
            );
        }
    }
}

Now here comes the issue. Whenever I have any other folder selected and I try to boot. I get an error The file that I will boot from Error But If I were to boot from the index file Booting from index Everything is working Everything is working properly. The problem is that in my course , In order for me to check that the username and password enters the database, I need to do some simple debugging and follow the code from the login controller. But I cant do that for some reason.

Im sorry that I do not know the proper terms for everything, I have a lot of gaps in my programming knowledge overall. If you could explain it to be with the proper terms while also explaining what those proper terms are exactly , that would be amazing.