After creating a new environment and database. I was facing this error. Even trying to logging in sys as sysdba was failing.
Looking into the error doesn't exactly tell what the issue is. This was actually environmental variable related issue. Nothing to do with tns, listener or even with the database.
I'm running oracle on linux. So needs to set ORACLE_HOME and ORACLE_SID variable correctly.
That's it! Now I'm able to login