This issue is most commonly caused by incorrectly configured SSL settings for your OS Commerce. If you don not have private SSL and your OS Commerce is enabled you will receive this error.

Enabling/disabling SSL in osCommerce is quite straightforward. You have to edit /includes/configure.php to disable SSL and the code takes care of the rest. This is an example of SSL enabled configuration:

// Define the webserver and path parameters
// * DIR_FS_* = Filesystem directories (local/physical)
// * DIR_WS_* = Webserver directories (virtual/URL)
define('HTTP_SERVER', 'http://www.yourdomain.com'); // eg, http://localhost - should not be empty for productive servers
define('HTTPS_SERVER', 'https://yourdomain.com'); // eg, https://localhost - should not be empty for productive servers
define('ENABLE_SSL', true); // secure webserver for checkout procedure?
define('HTTP_COOKIE_DOMAIN', 'www.yourdomain.com');
define('HTTPS_COOKIE_DOMAIN', 'yourdomain.com');
define('HTTP_COOKIE_PATH', '/catalog/');
define('HTTPS_COOKIE_PATH', '/catalog/');
define('DIR_WS_HTTP_CATALOG', '/catalog/');
define('DIR_WS_HTTPS_CATALOG', '/catalog/');

In order to disable the SSL you will have to replace

define('ENABLE_SSL', true);

with

define('ENABLE_SSL', false);