Such error occurs if 'register_globals' is disabled in /etc/php.ini, but osCommerce application requires PHP 'register_globals' option to be set to 'On'.
ResolutionPlease set 'php_admin_flag register_globals=On' in 'vhost.conf' for the domain osCommerce is installed on.
Or you may enable 'php_admin_flag=On' for whole server in/etc/php.ini, but it is not recommended due to security reasons.
See more inormation about vhost.conf at http://download1.swsoft.com/Plesk/Plesk8.1/Doc/plesk-8.1-unix-administrators-guide/17359.htm
Also 'register_long_arrays' option should be switched on to avoid the warning messages like "failed to open stream: No such file or directory".