ORA-02429
ORA-02429 means that the index you want to drop is a primary key.
SQL> drop index hr.JHIST_EMP_ID_ST_DATE_PK;
drop index hr.JHIST_EMP_ID_ST_DATE_PK
*
ERROR at line 1:
ORA-02429: cannot drop index used for enforcement of unique/primary key
Although primary key is an index, it's also a constraint. You should treat it like a constraint first, then drop it as an index.
Drop Constraint
The constraint name may or may not be the same as the index name, we should find it out.
SQL> column constraint_name format a30;
SQL> select constraint_name from all_constraints where index_owner = 'HR' and index_name = 'JHIST_EMP_ID_ST_DATE_PK';
CONSTRAINT_NAME
------------------------------
JHIST_EMP_ID_ST_DATE_PK
Then we drop the constraint.
SQL> alter table hr.job_history drop constraint JHIST_EMP_ID_ST_DATE_PK;
Table altered.
Drop Index
Then drop the index if any. I've talked more about this in How to Drop Primary Key.
SQL> drop index hr.JHIST_EMP_ID_ST_DATE_PK;
Index dropped.
The index has been dropped.