create or replace
PROCEDURE SP_IF334 
(
  AUD_ROWID IN VARCHAR2  
, FILE_PREFIX IN VARCHAR2  
, FILE_NAME OUT VARCHAR2  
) AS 
FoundGtransNum number(12);
fHandle  UTL_FILE.FILE_TYPE;
BEGIN
  dbms_output.put_line(' - IF334: Entered at '||to_char(sysdate,'dd/mm/yy hh24:mi:ss'));
  
  select gtrans_num
  into   FoundGtransNum
  from   user_ax.gtrans_trig
  where  rowid = AUD_ROWID;
  
  FILE_NAME := FILE_PREFIX||FoundGtransNum||'.csv';
  
  dbms_output.put_line(' - IF334: Filename: '||FILE_NAME);
  
  fHandle := UTL_FILE.FOPEN('AX_OUT', FILE_NAME, 'W');

  UTL_FILE.PUT_LINE(fHandle, 'This is the first line');
  UTL_FILE.PUT_LINE(fHandle, 'This is the second line');
  UTL_FILE.PUT_LINE(fHandle, 'This is the third line');

  UTL_FILE.FCLOSE(fHandle);
  
  
  dbms_output.put_line(' - IF334: Finished at '||to_char(sysdate,'dd/mm/yy hh24:mi:ss'));
END SP_IF334;

Error report:
ORA-29283: invalid file operation
ORA-06512: at "SYS.UTL_FILE", line 536
ORA-29283: invalid file operation
ORA-06512: at "USER_AX.SP_IF334", line 23
ORA-06512: at line 130
29283. 00000 -  "invalid file operation"
*Cause:    An attempt was made to read from a file or directory that does
           not exist, or file or directory access was denied by the
           operating system.
*Action:   Verify file and directory access privileges on the file system,
           and if reading, verify that the file exists.

        elsif found_if.if_ref = 'IF334' then
          --IF334: Stock Movement
          dbms_output.put_line(' - Calling IF334');
          dbms_output.put_line(' - audit_record.rowid      : '||audit_record.rowid);
          dbms_output.put_line(' - found_if.output_prefix  : '||found_if.output_prefix);
          user_ax.SP_IF334(audit_record.rowid,found_if.output_prefix,res_output_filename);
          dbms_output.put_line(' - Procedure SP_IF334 completed. Filename returned is '''||res_output_filename||'''');

 - Calling IF334
 - audit_record.rowid      : AAAh6+AAvAAEmWkAAM
 - found_if.output_prefix  : 334_StkMove_
 - IF334: Entered at 15/04/14 10:26:12
 - IF334: Filename: 334_StkMove_8122560.csv

...
fHandle := UTL_FILE.FOPEN('AX_OUT', FILE_NAME, 'W');
...

select * from dba_directories where directory_name = 'AX_OUT';

OWNER   DIRECTORY_NAME   DIRECTORY_PATH
SYS	AX_OUT	         /procdi/tmpeoU/AX/

select grantor, 
       grantee, 
       table_schema, 
       table_name, 
       privilege
from   all_tab_privs 
where  grantee = 'USER_AX';

GRANTOR     GRANTEE     TABLE_SCHEMA    TABLE_NAME    PRIVILEGE 
----------- ----------- --------------- ------------- ------------ 
PRO         USER_AX     SYS             AX_OUT        EXECUTE 
PRO         USER_AX     SYS             AX_OUT        READ 
PRO         USER_AX     SYS             AX_OUT        WRITE 

...
FILE_NAME := FILE_PREFIX||FoundGtransNum||'.csv';
sys.dbms_output.put_line('The file name is: ' || FILE_NAME);

 - Calling IF334
 - audit_record.rowid      : AAAh6+AAvAAEmWkAAM
 - found_if.output_prefix  : 334_StkMove_
 - IF334: Entered at 15/04/14 11:45:24
The file name is: 334_StkMove_8122560.csv
The file prefix is: 334_StkMove_
The FoundGtransNum is: 8122560
 - IF334: Filename: 334_StkMove_8122560.csv