SQL & PL/SQL

Hello,
The procedure I am running within sqlplus is not displaying the output of the dbms_output.put_line to the screen. I have read quite a bit of posts on this forum, but still cannot understand what the workaround is to it. This is a snippet of my code:
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set echo off feedback off verify off
set serveroutput on

connect sys/&SYSPASS1 as sysdba;

   BEGIN
   CASE '&2.'
    WHEN 'user'
       THEN do something
       ELSE 
      dbms_output.enable(5000);
      dbms_output.put_line('Error: Invalid command');
    END CASE;
  END;
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