Oracle Message ORA-26027 - unique index string.string partition string initially in unusable state

ERRORunique index string.string partition string initially in unusable state
CAUSEA partition of a unique index is in IU state (a unique index * cannot have index maintenance skipped via SKIP_UNUSABLE_INDEXES).
ACTIONEither rebuild the index or index partition, or use * SKIP_INDEX_MAINTENANCE if the client is SQL*Loader.