404 not found!

I'm starting to suspect that you get a true 404 error message and not just Codeigniters 404.

Just to clarification, can you access your user/login with the following URL:
localhost/user/login

If not, does it work with the following URL?
localhost/index.php/user/login

If it does, you need to do these steps to get rid of index.php
https://www.codeigniter.com/user_guide/g...x-php-file

After that the following url should start working:
localhost/subit_backend/register

Instead of:
localhost/index.php/subit_backend/register