The specified partitioning foreign key was not supported for reference-partitioned tables. All columns of the partitioning foreign key must be constrained NOT NULL with enabled, validated, and not deferrable constraints. Furthermore, a virtual column cannot be part of the partitioning foreign key.
Reissue the command after ensuring that the two subpartitions being merged lie in the same composite partition.
Try the following links for this error:
http://docs.oracle.com/cd/E16655_01/server.121/e26079/e12700.htm
https://community.oracle.com/message/10105276
https://community.oracle.com/message/11049887
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