SVRMGR> connect INTERNAL@orcl
Password:
ORA-12638: Credential retrieval failed

If you get this error first you need to check the SQLNET.ORA file.

This file is located in [Oracle_Home]/NETWORK/ADMIN/SQLNET.ORA

if you see

SQLNET.AUTHENTICATION_SERVICES= (NTS)

this need to change as below

SQLNET.AUTHENTICATION_SERVICES= (NONE)