ORA-29283: invalid file operation ORA-06512: at “SYS.UTL_FILE”, line 536 ORA-29283: invalid file operation

Connected to Oracle database through service name, then get errors by running the below sample code. There is no problem to run the sample code while connecting to database through IPC.

$sqlplus testuser/****@DBTEST

SQL>DECLARE
    l_output utl_file.file_type;
BEGIN
    l_output := utl_file.fopen( 'MY_DIR', 'test.txt', 'w' );
    utl_file.fclose( l_output ); 
END;
/

*
ERROR at line 1:
ORA-29283: invalid file operation
ORA-06512: at "SYS.UTL_FILE", line 536
ORA-29283: invalid file operation
ORA-06512: at line 4

$sqlplus  / as sysdba
SQL> DECLARE
     l_output utl_file.file_type;
BEGIN
     l_output := utl_file.fopen( 'MY_DIR', 'test.txt', 'w' );
     utl_file.fclose( l_output );
END;

 /

PL/SQL procedure successfully completed.