That message means that your IdP (Shibboleth) server has not been configured to work with ClearPass as a service provider (SP).
You need to export the SP metadata from ClearPass, and import it into your IdP for authorization. Also configure your IdP to return the username in the attribute named 'principal'.
There is a recent technote on how to configure Single Sign On on ClearPass. It is available for partners at:
https://afp.arubanetworks.com/afp/index.php/ClearPass_Technical_Information_Links
If not running in a lab environment, please get your Shibboleth administrator involved.