What is causing 404 error in CodeIgniter?

to remove index.php from URL you do not have to change in .htacess

you can achieve the same just by changing in

PHP Code:

/app/Config/App.php 
public string $indexPage = ''; 


although if you still wanna do it using .htaccess and if your server supports rewriting URLs you can easily remove this file with URL rewriting.
you can go through official docs. removing-the-index-php-file

Learning Codeigniter