Begin
Merge INTO Storedgrades Sg USING
(SELECT Co.Credittype FROM Course Co
) ON ((Sg.Course_Number=Co.Course_Number) AND (NVL(Sg.Credit_Type,-1) <>Co.Credittype) )
WHEN Matched THEN
  Update Set Sg.Credit_Type=Co.Credittype
  --where (NVL(Sg.Credit_Type,-1) <>Co.Credittype);
End

create table courses_test
(	dcid number(10,0) constraint courses_pk primary key, 
	id number(10,0), 
	course_number varchar2(11 char), 
	code varchar2(20 char), 
	credittype varchar2(20 char) 
);

create table storedgrades_test 
(	dcid number(10,0) constraint storedgrades_pk primary key, 
	studentid number(10,0), 
	termid number(10,0), 
	storecode varchar2(10 char),
	course_number varchar2(11 char),
	credit_type varchar2(20 char) 
);

insert into courses_test( dcid, id, course_number, code, credittype ) values( 1, 1, 'AAA', 'Test 1', 1 );
insert into courses_test( dcid, id, course_number, code, credittype ) values( 2, 1, 'BBB', 'Test 1', 1 );
insert into courses_test( dcid, id, course_number, code, credittype ) values( 3, 1, 'BBB', 'Test 3', 3 );
insert into courses_test( dcid, id, course_number, code, credittype ) values( 4, 1, 'BBB', 'Test 5', 5 );

insert into storedgrades_test( dcid, studentid, termid, storecode, course_number, credit_type ) values( 1, 1, 1, 'Test 1', 'AAA', 1 );
insert into storedgrades_test( dcid, studentid, termid, storecode, course_number, credit_type ) values( 2, 1, 1, 'Test 2', 'AAA', 2 );
insert into storedgrades_test( dcid, studentid, termid, storecode, course_number, credit_type ) values( 3, 1, 1, 'Test 1', 'BBB', 1 );
insert into storedgrades_test( dcid, studentid, termid, storecode, course_number, credit_type ) values( 4, 1, 1, 'Test 4', 'BBB', 4 );

commit;

select * from courses_test;

      DCID         ID COURSE_NUMB CODE                 CREDITTYPE
---------- ---------- ----------- -------------------- --------------------
         1          1 AAA         Test 1               1
         2          1 BBB         Test 1               1
         3          1 BBB         Test 3               3
         4          1 BBB         Test 5               5

select * from storedgrades_test;

      DCID  STUDENTID     TERMID STORECODE  COURSE_NUMB CREDIT_TYPE
---------- ---------- ---------- ---------- ----------- --------------------
         1          1          1 Test 1     AAA         1
         2          1          1 Test 2     AAA         2
         3          1          1 Test 1     BBB         1
         4          1          1 Test 4     BBB         4

MERGE INTO storedgrades_test sg
USING courses_test co
ON ((sg.course_number=co.course_number)
    AND (nvl(sg.credit_type,-1) <>co.credittype))
WHEN MATCHED THEN UPDATE SET sg.credit_type=co.credittype;

ERROR at line 3:
ORA-38104: Columns referenced in the ON Clause cannot be updated:
"SG"."CREDIT_TYPE"

UPDATE storedgrades_test sg
SET sg.credit_type=
  ( SELECT co.credittype
    FROM courses_test co
    WHERE sg.course_number=co.course_number
      AND nvl(sg.credit_type,-1) <>co.credittype )
WHERE EXISTS
  ( SELECT co.course_number
    FROM courses_test co
    WHERE sg.course_number=co.course_number
      AND nvl(sg.credit_type,-1) <>co.credittype );

ERROR at line 3:
ORA-01427: single-row subquery returns more than one row