A weak entity with many to many relationship with its owner SQL
The FOREIGN KEY is not correct. You must create 2 of them. One the PATIENT table for the FILE_NO field and one on the APPOINTMENT table for the APP_NO field.

PAT_TO_FK FOREIGN KEY APT_PATIENT_FK (FILE_NO) REFERENCES PATIENT(FILE_NO)
APP_TO_FK FOREIGN KEY APT_APPOINTMENT_FK (APP_NO) REFERENCES APPOINTMENT(APP_NO)