just installed but the first page that appears 404 not found

Please post the config.php file located inside config folder in your trongate web app.
Secondly tell me the link that you pasted in browser which gave you the error.
:)

If you have installed the Trongate framework from github you must set the BASE_URL in the config.php to something like 'http://localhost/your_app_name/'
The forward slash at the end is important, so don't forget it.

// config.php
define('BASE_URL', 'http://localhost/demo_code/');
define('ENV', 'dev');
define('DEFAULT_MODULE', 'welcome');
define('DEFAULT_CONTROLLER', 'Welcome');
define('DEFAULT_METHOD', 'index');
define('MODULE_ASSETS_TRIGGER', '_module');

// URL
http://localhost/demo_code

is there something wrong ?

That config looks good yoesakae.

This may sound silly, but did you save config.php when you changed BASE_URL to 'http://localhost/demo_code/'

What OS you are using, Mac, Windows or Linux?
What stack are you using (Xampp, Wamp, Lamp etc)?
What is the folder name of your app?

Your app name should also be demo_code as you set in the BASE_URL if you downloaded trongate framework from github.
Does the 404 page appear in red background or white?

I'm on Windows 10 + Xampp and put "demo_code" in the HtDocs folder.

I have installed repeatedly via trongate.exe but the problem remains the same "404 Not Found".

installing from trongate.exe... do you mean the desktop app and not a manual install from github?

Also, the '404 page not found' error message - is it a red screen with white text?

I downloaded from the following link: https://trongate.io/download

exactly the same as laravel error message "404 not found" with gray background.

try to change http to https