Oracle parent key not found

I am trying to add a constraint to the booking table, reservations can be for flight or accommodation or both.

• First 4 entries of booked domestic flights, departure and accommodation
• The next 4 entries have booked a flight only and have acc_id
  
  forNULL
  
  
• After two entries, only accommodation is booked, so in-flight, out-of-flight, and seats are set to null.

Here are my limits for this table

ALTER TABLE HOLIDAY_RESERVATION ADD CONSTRAINT HOLIDAY_PK PRIMARY KEY (RESV_ID);
ALTER TABLE HOLIDAY_RESERVATION ADD CONSTRAINT CUSTOMER_FK FOREIGN KEY (BOOKING_CUS_ID) REFERENCES CUSTOMER (CUS_ID);
ALTER TABLE HOLIDAY_RESERVATION ADD CONSTRAINT STAFF_FK3 FOREIGN KEY (EMP_ID) REFERENCES STAFF (EMP_ID);
ALTER TABLE HOLIDAY_RESERVATION ADD CONSTRAINT FLIGHT_FK FOREIGN KEY (IN_FLIGHT_ID) REFERENCES FLIGHT (FLI_ID);
ALTER TABLE HOLIDAY_RESERVATION ADD CONSTRAINT FLIGHT_FK1 FOREIGN KEY (OUT_FLIGHT_ID) REFERENCES FLIGHT (FLI_ID);
ALTER TABLE HOLIDAY_RESERVATION ADD CONSTRAINT ACC_FK FOREIGN KEY (ACC_ID) REFERENCES ACCOMMODATION (ACC_ID);

      

        
        
        
      

    

and the only limitation that gives the error is:

ALTER TABLE HOLIDAY_RESERVATION ADD CONSTRAINT FLIGHT_FK1 FOREIGN KEY (OUT_FLIGHT_ID) REFERENCES FLIGHT (FLI_ID);

      

        
        
        
      

    

I get

ERROR at line 1:
ORA-02298: cannot validate (U1146815.FLIGHT_FK1) - parent keys not found

      

        
        
        
      

    

What's the problem? I understand that it has to do with orphans, but I am setting zeros, so I don't understand, please advise