Application is running inside IIS process but is not configured to use IIS server .NET Core 3.0

I also came across this issue while following the documentation https://docs.microsoft.com/en-us/aspnet/core/migration/22-to-30?view=aspnetcore-3.0&tabs=visual-studio

For your case I have checked and the code below will work, with the call to webBuilder.UseKestrel() removed.

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

In my case I ran wrong profile (IIS Express) in Visual Studio by carelessness.

I had the same error then to fix my problem I needed to change webBuilder.UseKestrel(); to ConfigureKestrel(serverOptions => {})

My Program.cs before:

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

My Program.cs fixed:

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