SP2-0158 means that the issued command is not supported by this release of SQL*Plus utility. For example, we use SQL*Plus 188.8.131.52 to connect to a multitenant database like this.
C:\Users\edchen>sqlplus sys/password@orclcdb as sysdba
SQL*Plus: Release 184.108.40.206.0 Production on Wed Feb 3 19:37:44 2019
Copyright (c) 1982, 2013, Oracle. All rights reserved.
Oracle Database 18c Enterprise Edition Release 220.127.116.11.0 - Production
We'd like to know what PDB we have by issuing SHOW PDBS.
SQL> show pdbs;
SP2-0158: unknown SHOW option "pdbs"
Since Oracle multitenant databases starts from 12c, so there's no way to know the new feature for SQL*Plus 18.104.22.168.0.
For the same reason, all multitenant related commands like SHOW CON_NAME cannot be used, either.
Use higher releases of SQL*Plus or SQL Developer to issue the command. In my opinion, Oracle instant client is the fastest way to make it work.