What do do if SET SERVEROUTPUT ON doesn't work

If you have an anonymous piece of pl/sql code and you have turned on serveroutput, like this:

set serveroutput on

You may need to enable output first:

exec dbms_output.enable();
set serveroutput on

In a multitenant environment, you will need to enable serveroutput after every time you switch container.

So, if you do

BEGIN
V_SQLSTRING := 'ALTER SESSION SET CONTAINER=' || '' || V_PDBNAME || '';
DBMS_OUTPUT.PUT_LINE(V_SQLSTRING);
EXECUTE IMMEDIATE V_SQLSTRING;
END;
/

you will need to repeat the enabling of serveroutput again, if you want to perform addition actions further down in your script.