After you are connected with sqlplus to a container database (which is happening by default), run the connect command inside your sqlplus session.

conn SYS@PMJ AS SYSDBA

Then you'll be required to enter the SYS password.

Another way to switch to a PDB is to change the container:

ALTER SESSION SET container=PMJ;

To go back to the CDB container:

ALTER SESSION SET container=CDB$ROOT;

sqlplus /nolog
connect SNT@PMJ