Asp.net core default route

routes.MapRoute(
    name: "default",
    template: "{controller}/{action}/{id?}",
    defaults: new { controller = "Main", action = "Index" });

routes.MapRoute(
    name: "default",
    template: "{controller=Main}/{action=Index}/{id?}");

These are the two ways of defining default route. You are mixing them. You need always to define a template. In the second way you can write the defaults directly in the template.

Another solution would be to redirect "/" to another url

 app.UseEndpoints(endpoints =>
        {
            endpoints.MapDefaultControllerRoute();

            endpoints.MapGet("/", context =>
            {
                return Task.Run(() => context.Response.Redirect("/Account/Login"));
            });
        });

The easiest way for me (and without using MVC) was to set the controller to default route using empty [Route("")] custum attribute like so:

[ApiController]
[Route("")]
[Route("[controller]")]
public class MainController : ControllerBase
{ ... }

with Startup.Configure

app.UseRouting();
app.UseEndpoints(endpoints =>
{
    endpoints.MapControllers();
});