404 Not Found on CSS file even though it’s in the correct directory

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I don’t think using ROOT_PATH is what you want to do here. That loads the server root.

try this code:

//add some pizazz
function pt_load_plugin_css () {
    
    $plugin_url = plugin_dir_url( __FILE__ );
    wp_enqueue_style ('pt-style', $plugin_url . 'css/pt.css', '1.0.0');
}
add_action ('wp_enqueue_scripts', 'pt_load_plugin_css');