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ORA-12717:

Cannot issue ALTER DATABASE NATIONAL CHARACTER SET when NCLOB, NCHAR or NVARCHAR2 data exists
Cause: NCLOB, NCHAR or NVARCHAR2 data changed the representation to Unicode when converting to a multibyte character set and must be migrated.
Action: Remove NCLOB, NCHAR or NVARCHAR2 data as listed in the alert file. The above type data can be migrated by methods such as import/export.