Client certificate is required. No certificate was found in the request. This might be because the client certificate could ...

Client certificate is required.  No certificate was found in the request.  This might be because the client certificate could not be successfully validated by the operating system or IIS.  For information on how to bypass those validations and use a custom X509CertificateValidator in WCF please see http://go.microsoft.com/fwlink/?LinkId=208540.