Unable to connect sys as sysdba locally

@DBALIAS"

here is my environment

1. Oracle 12.2
2. NON-CDB
3. The oracle user is part of dba and oinstall group as follows$ id oracle uid=1000(oracle) gid=54322(dba) groups=54322(dba),54321(oinstall)
4. The ORAPW file has been created using ORAPWD utility
5. ORACLE_HOME and ORACLE_SID set to DBALIAS

Here is my problem:

I can connect using following meethods

a. sqlplus SYS"@DBALIAS" as sysdba ( it asks for password )

b. sqlplus SYS/PW@DBALIAS as sysdba

But when I use following, I get ORA-12154 error

sqlplus '/ as sysdba'

ERROR: ora-12154: TNS: could not resolve the connect identifier specified