the Business Connector imports all IDoc metadata from the SAP system’s DDIC. If the IDoc type does not exist in the SAP system, then the BC won’t be able to lookup the metadata and create the correct RFC format for sending it into SAP.
Solution: create the correct IDoc defintion for your Z-type in the SAP backend, using transactions WE30 (IDoc types) and WE31 (segment types).
Note: if the SAP system already has the definition for your Z-IDoc, then it is most probably an authorization problem. (You can test this by doing an “IDoc Lookup” from the BC side using a standard SAP IDoc like ORDERS01. This type is guaranteed to exist, so if you get the same problem with that one as well, then you know your user doesn’t have the necessary permission for the DDIC lookup. See SAP note 460089 for the necessary authorization objects.)