Please see
http://www.cisco.com/c/en/us/td/docs/security/ise/2-0/api_ref_guide/api_ref_book/ise_api_ref_guest.html#55905
Response Error Messages
You are required to use certain headers when you do a POST or PUT request. You will get errors if you leave one out.
The log files that contains detailed messages that can enable you to correct the operations can be found in the Cisco ISE UI at the following location:
Operations > Node > Debug Logs > guest.log