file_exists function does not work

file_exists() lets you check if a file exists on the local server, by passing it a file path. It cannot be used to check for the existence of files via URL, and you’re passing it the result of get_stylesheet_directory_uri(), which returns the URL (http:// etc.) to the file, not the path.

The proper way, these days, to get the path to a theme file is to use get_theme_file_path(), like so:

$filecs = get_theme_file_path( 'css/lucilevi.css' );
$filejs = get_theme_file_path( 'js/lucilevi.js' );

if ( file_exists( $filecs ) ) {
    // etc.
}

if ( file_exists( $filejs ) ) {
    // etc.
}

Just be aware that because $filecs and $filejs are file paths, you cannot pass them to wp_enqueue_style(), wp_enqueue_script(), you still need to pass URLs to those.