Hi
You mean the 'system.css' file, right? In previous versions of Joomla, a template will call this css file in default system template of Joomla 3, however, in Joomla 4, this template was removed so it notices that error message.
I just updated the head block of template: /templates/ja_aiga/tpls/blocks/head.php
<?php
// SYSTEM CSS
$this->addStyleSheet(JUri::base(true) . '/templates/system/css/system.css');
?>
change to:
<?php
if (version_compare(JVERSION, '4.0', 'lt')) {
// SYSTEM CSS
$this->addStyleSheet(JURI::base(true) . '/templates/system/css/system.css');
}
?>
It's ok now.