D:\distrib\ora21\network\admin>sqlplus admin@DB

SQL*Plus: Release 21.0.0.0.0 - Production on Fri Oct 15 14:35:32 2021
Version 21.3.0.0.0

Copyright (c) 1982, 2021, Oracle.  All rights reserved.

Enter password:
ERROR:
ORA-28759: failure to open file


D:\distrib\ora21\network\admin>type sqlnet.ora
# sqlnet.ora Network Configuration File: c:\ora10\network\admin\sqlnet.ora
# Generated by Oracle configuration tools.

# This file is actually generated by netca. But if customers choose to
# install "Software Only", this file wont exist and without the native
# authentication, they will not be able to connect to the database on NT.

SQLNET.AUTHENTICATION_SERVICES= (NTS)

NAMES.DIRECTORY_PATH= (TNSNAMES, EZCONNECT)

WALLET_LOCATION = (SOURCE = (METHOD = file) (METHOD_DATA = (DIRECTORY="D:/distrib/ora21/network/admin")))
SSL_SERVER_DN_MATCH=yes

D:\distrib\ora21\network\admin>sqlplus admin@DB

SQL*Plus: Release 21.0.0.0.0 - Production on Fri Oct 15 14:37:59 2021
Version 21.3.0.0.0

Copyright (c) 1982, 2021, Oracle.  All rights reserved.

Enter password:
Last Successful login time: Fri Oct 15 2021 12:37:07 +03:00

Connected to:
Oracle Database 21c Enterprise Edition Release 21.0.0.0.0 - Production
Version 21.2.0.0.0

SQL> show parameter pga;

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
pga_aggregate_limit                  big integer 6000M
pga_aggregate_target                 big integer 3000M
SQL>