For that i put in the following code for trigger KEY-NEXT-ITEM for Cust_Code:
DECLARE
  FD_NO NUMBER;
  FD_AMT NUMBER;
  OPN_DT DATE;
  MAT_DT DATE;
BEGIN
  SELECT ACCT_FD_NO, AMOUNT, ACCT_OPN_DT, MATURITY_DATE INTO FD_NO, FD_AMT, OPN_DT, MAT_DT
  FROM KEC_FDACCT_MSTR WHERE ACCT_CUST_CODE = :CUST_CODE;
 
  :ACCT_FD_NO := FD_NO;
  :FD_AMOUNT := FD_AMT;
  :ACCT_OPN_DATE:= OPN_DT;
  :MATURITY_DATE:= MAT_DT;
END;

SQL> select ename from emp where deptno = 20 order by ename;

ENAME
----------
ADAMS
FORD
JONES
SCOTT
SMITH

SQL> DECLARE
  2     l_ename   VARCHAR2 (15);
  3  BEGIN
  4     FOR cur_r IN (  SELECT ename
  5                       FROM emp
  6                      WHERE deptno = 20
  7                   ORDER BY ename)
  8     LOOP
  9        l_ename := LTRIM (SUBSTR (l_ename || ', ' || cur_r.ename, 1, 15), ', ');
 10     END LOOP;
 11
 12     DBMS_OUTPUT.put_line ('DEPTNO 20 employees: ' || l_ename);
 13  END;
 14  /
DEPTNO 20 employees: ADAMS, FORD, JO

PL/SQL procedure successfully completed.

SQL>

DECLARE 
    l_fd VARCHAR2 (1000); 
BEGIN 
    FOR cur_r IN (SELECT acct_fd_no 
                  FROM   kec_fdacct_mstr 
                  WHERE  staus = 'E' 
                  AND acct_cust_code = '336' 
                  ORDER  BY acct_fd_no) LOOP 
        l_fd := cur_r.acct_fd_no; 

        dbms_output.Put_line ('FD336:  ' || l_fd); 
    END LOOP; 
END; 

DECLARE 
    l_fd VARCHAR2 (1000); 
BEGIN 
    FOR cur_r IN (SELECT acct_fd_no 
                  FROM   kec_fdacct_mstr 
                  WHERE  staus = 'E' 
                  AND acct_cust_code = :CUST_CODE) LOOP 
        l_fd := cur_r.acct_fd_no; 

        :ACCT_FD_NO := l_fd; 

        next_record; 
    END LOOP; 
END; 

    FOR cur_r IN (SELECT acct_fd_no 
                  FROM   kec_fdacct_mstr 
                  WHERE  staus = 'E' 
                  AND acct_cust_code = :CUST_CODE
                  AND acct_fd_no IS NOT NULL            --> add this line
                 ) LOOP 

BEGIN 
    FOR cur_r IN (SELECT acct_fd_no,
                         cust_code                     --> here ...
                  FROM   kec_fdacct_mstr 
                  WHERE  staus = 'E' 
                  AND acct_cust_code = :CUST_CODE
                 ) 
    LOOP 
       :ACCT_FD_NO := cur_r.acct_fd_no;
       :CUST_CODE  := cur_r.cust_code;                 --> ... and here

        next_record; 
    END LOOP; 
END; 

FOR CUR_R IN (SELECT  ACCT_FD_NO FROM KEC_FDACCT_MSTR 
              WHERE   STAUS='E' 
              AND ACCT_CUST_CODE=:CUST_CODE  
              ORDER BY ACCT_FD_NO)
       LOOP
          :ACCT_FD_NO :=CUR_R.ACCT_FD_NO;
          MESSAGE('FD: '|| CUR_R.ACCT_FD_NO);
          MESSAGE(' ');
          NEXT_RECORD;
       END LOOP;

loop
  ...
  create_record;        --> instead of next_record
end loop;

BEGIN
FOR CUR_R IN (SELECT  ACCT_FD_NO FROM KEC_FDACCT_MSTR
              WHERE   STAUS='E' 
              AND ACCT_CUST_CODE=:CUST_CODE 
              ORDER BY ACCT_FD_NO)
LOOP
   :ACCT_FD_NO :=CUR_R.ACCT_FD_NO;
   NEXT_RECORD;
END LOOP;
END;