Your web server could not open includes/sql_layer.php. There are many reasons for this (Fatal error: Failed opening required includes/sql layer.php):
The file is really not there, something went wrong with the installation - reinstall (see Chapter 3).
The file is there but the web server does not have the right to open it - check permissions (see Section 3.2.3).
There is a .htaccess file somewhere in the directory hierarchy that denies access to you.
There is something wrong with the include path of the PHP interpreter (see Section 3.9.10).
You might see more information on the cause of your problem if you have access to the web server access and error logs.
PHP-Nuke HOWTO adapted for PHP-Nuke by Chris Karakas