Error while creating relationship - "Choose the key from allowed namespace"

Scenario	You are trying to enhance standard data model by creating new entity types and creating relationships.
Issue	Error when trying to create a leading relationship for new entity type (SU type-4) on entity type-1 .
Error Screenshot
Solution	Create new relationship by prefixing ZZ i.e., ZZMATMARC1

MDG - Web Workbench for Data Modeling

You can use MDG's Web Workbench to do normal MDG data modeling tasks using below url

http://<host>:<ITS-Port>/sap/bc/webdynpro/sap/wda_mdg_dt_conf_workbench?FROM_TRANS=X&sap-client=800&sap-language=EN#