How to do a SELECT with a composite key?

I received the following error while trying to enable constraint OBL_TXN_PROJECT_PHASE_FK:

ALTER TABLE PRODUCTION.OBLIGATION_TRANSACTION

  MODIFY CONSTRAINT OBL_TXN_PROJECT_PHASE_FK

  ENABLE

Error at line 2

ORA-02298: cannot validate (PRODUCTION.OBL_TXN_PROJECT_PHASE_FK) - parent keys not found

The  FOREIGN KEY of CONSTRAINT OBL_TXN_PROJECT_PHASE_FK  is  (PROJECT_ID, PHASE_NUMBER)

which REFERENCES Parent table PROJECT_PHASE (PROJECT_ID,PHASE_NUMBER)

I am trying to locate "orphan rows" with SQL similar to the following:

select child_key from from child_table
where
child_key not in (select parent_key from parent_table);