You might have seen this error during extension or custom development for MDM (v10/v11.x). After code generation, it creates sample requests in workbench. By default, the xsd file in xml schema in generated sample request is MDMCommon.xsd, which does not have newly developed extension data types definitions. So you need to change it to MDMDomains.xsd.
After changing it try to run sample request as MDM transaction, and it should work. As usual you need to deploy the application, run sqls and recycle hub.
For more information see the following link from IBM: