Oracle SQL 12c How to correctly use compound foreign keys in compound primary keys?
ORA-02256: number of referencing columns must match referenced columns
  
  Cause: The number of columns in the foreign-key referencing list is not equal to the number of columns in the referenced list.
  Action: Make sure that the referencing columns match the referenced columns.


The problem is the foreign key should reference both columns used in the composite primary key.

Add the FLIGHT_DATE column to the RESERVATIONS table and change your code:

CONSTRAINT FK_reservations_flight_ID FOREIGN KEY (FLIGHT_ID)
    REFERENCES FLIGHT(FLIGHT_ID,FLIGHT_DATE)
);


To:

CONSTRAINT FK_reservations_flight_ID FOREIGN KEY (FLIGHT_ID,FLIGHT_DATE)
    REFERENCES FLIGHT(FLIGHT_ID,FLIGHT_DATE)
);