If you face this error while creating materialized view:
ORA-30373: Object Data Types Are Not Supported In This Context
This means you are trying to create a Materialized View that references columns data types such as XMLTYPE or SDO_GEOMETRY or ESRI ST_GEOMETRY. This is a 12c limitation.
On the other hand, if you try to crate a materialized view with ST_GEOMETRY column this won’t be allowed.The workaround is not including the ST_GEOMETRY attribute in the materialized view.