give us error please
What OP posted is the error given by SQL Developer which is what OP is probably using and they possibly already got the solution.
Sql developer will give that message if you use DBMS_METADATA but don't have the select_catalog_role or try to use objects owned by another user.
See Joyce's reply in this thread from two days before OPs post Re: Problems Exporting Tables to DDL
If you do not have select_catalog_role and export objects owned by another schema, the export will attempt to use the internal generator which is no longer supported. DDL generated by the internal generator may not be correct or complete.