E-signature error in RTC 4.0.

I have setup an e-signature feature in one of my custom work item type. When the precondition is met , the user logged in is trying to sign but it error out saying

"Exception checking precondition. AN unhandled exception occured during "E-signature". An error response was received from the Jazz Team Server. Status=400. Message: CRJAZ0799W The external user directory does not support the requested feature.

Note: For this RTC server ( sandbox env), I have configured Non-LDAP external user registry as a means of user authentication.