What is causing 404 error in CodeIgniter?

I am attempting to install the latest version of CodeIgniter in a subdirectory, which has been successful thus far. However, I now aim to eliminate "index.php" from the URL. As a result, I have created an .htaccess file in the subfolder with the following directives:

RewriteEngine On
RewriteBase /dev/

# Redirects trailing slashes to prevent SEO duplicate content issues
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.+)/$ $1 [L,R=301]

# Disallows access to the system and application folders by users
# This enables the creation of a System.php controller, which was previously impossible
# Replace 'system' if your system folder has been renamed
RewriteCond %{REQUEST_URI} ^system.*
RewriteCond %{REQUEST_URI} ^application.*
RewriteRule ^(.*)$ /index.php/$1 [L]

# Checks if the user is attempting to access a valid file, such as an image or CSS document
# If not, the request is redirected to index.php
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /index.php/$1 [L]
The name of the subdirectory is "dev", and the webroot (parent folder) is empty, containing only an index.html file. The resulting file structure is as follows:

bash
/var/www/
    index.html
    dev/
        CodeIgniter installation here
        .htaccess
Mod-rewrite is enabled, but I am receiving a 404 error when attempting to call URLs without "index.php". Can you provide any insight into why this may be happening?