Trying to create a table with foreign keys and its missing a few rows This is what I have so far:

CREATE TABLE GRADE
(    
    COURSE_NO   VARCHAR2(8),
    STUDENT_ID  VARCHAR2(15), 
    GRADE   VARCHAR2(3), 
    PRIMARY KEY(COURSE_NO,STUDENT_ID) ENABLE
);

ALTER TABLE GRADE 
ADD FOREIGN KEY (COURSE_NO)
REFERENCES COURSE (COURSE_NO)ENABLE;

ALTER TABLE GRADE 
ADD FOREIGN KEY (STUDENT_ID)
REFERENCES STUDENT (STUDENT_ID) 
ON DELETE CASCADE ENABLE;

INSERT INTO GRADE(COURSE_NO,STUDENT_ID,GRADE) 
    VALUES ('CS100','123','B');  
INSERT INTO GRADE(COURSE_NO,STUDENT_ID,GRADE)   
    VALUES('CS100','234','A');   
INSERT INTO GRADE(COURSE_NO,STUDENT_ID,GRADE)   
    VALUES('CS100','456','B');   
INSERT INTO GRADE(COURSE_NO,STUDENT_ID,GRADE)   
    VALUES('CS100','567','A');   
INSERT INTO GRADE(COURSE_NO,STUDENT_ID,GRADE)   
    VALUES('CS200','678','B');   
INSERT INTO GRADE(COURSE_NO,STUDENT_ID,GRADE)   
    VALUES('CS200','789','C');
INSERT INTO GRADE(COURSE_NO,STUDENT_ID,GRADE)   
    VALUES('CS200','890','A');
INSERT INTO GRADE(COURSE_NO,STUDENT_ID,GRADE)   
    VALUES('CS200','901','A');
INSERT INTO GRADE(COURSE_NO,STUDENT_ID,GRADE)   
    VALUES('EE100','012','B');
INSERT INTO GRADE(COURSE_NO,STUDENT_ID,GRADE)   
    VALUES('EE100','013','A');  
INSERT INTO GRADE(COURSE_NO,STUDENT_ID,GRADE)   
    VALUES('EE100','014','C');   
INSERT INTO GRADE(COURSE_NO,STUDENT_ID,GRADE)   
    VALUES('EE100','015','B');  


INSERT INTO GRADE(COURSE_NO,STUDENT_ID,GRADE)   
    VALUES('EE220','016','B');           -- This one doesn't work

INSERT INTO GRADE(COURSE_NO,STUDENT_ID,GRADE)   
       VALUES('EE200','017','A');            

INSERT INTO GRADE(COURSE_NO,STUDENT_ID,GRADE)   
    VALUES('EE200','018','B');           -- this one either


INSERT INTO GRADE(COURSE_NO,STUDENT_ID,GRADE)   
       VALUES('MA220','019','B');

INSERT INTO GRADE(COURSE_NO,STUDENT_ID,GRADE)   
     VALUES('MA220','020','B');                     


INSERT INTO GRADE(COURSE_NO,STUDENT_ID,GRADE)   
    VALUES('MA220','021','B');           -- This insert doesn't work
The foreign keys are defined on course_no and student_id. Assuming the error is a foreign key violation (big assumption, please post the full error in future questions) it must be one of those two at fault. Other rows reference course EE200 so it's not that; so student 018 doesn't exist in the student table. Similarly MA220 so student 021 is missing. With EE220 / 016 there are no comparators so either or both could be missing. Given your record I'd suggest the student.

This is basic stuff and fundamental to the concept of foreign keys. You should read up on the concept and ensure you write data in a sequence that respects the constraints you have defined. Otherwise you are in for a world of pain.