ORA-12638 is a server side error indicating that the client either Oracle SQL Plus, Forms, Designer or any other client software was unable to connect to the database. This issue is primarily noticed in environments that rely on NTS based authentication and is known to be resolved after bouncing the database services or the database server. The best way to figure out the exact issue causing this error is to turn the trace on.
If you are receiving ORA-12638 on a first time authentication and your database administrator has not specifically asked you to use NTS authentication, the issue could be with your SQLNET.ORA Authentication services parameter. To fix this issue, open your Sqlnet.ORA file and look for a line that has "SQLNET.AUTHENTICATION_SERVICES=(NTS)" and change that to "SQLNET.AUTHENTICATION_SERVICES=(NONE)"
ORA-12638 Documentation :
ORA-12638: Credential retrieval failed
Cause: The authentication service failed to retrieve the credentials of a user.
Action: Enable tracing to determine the exact error.