HI Kostas.

1. Edit that file: templates\investa\error.php
2. Then after custom code changes - change file permission from 644 to 444.

Extra tip!

Inside that Error page you can publish any module you want using "404" module position. It will be displayed under Error message.

Ok and how i can see the default error page so i can edit it? Can you guide me if possible, i want the error page to redirect to sp page builder page. I have sef enabled. Thanks in advance.

Sorry, but php customization is beyond support.

I gave you basic tips already. Maybe contact with local PHP Joomla Developer.