Unable to get the DBMS Logs in Oracle SQl Developer
This works just fine:

SET SERVEROUTPUT ON
BEGIN
  DBMS_OUTPUT.PUT_LINE('Test);
END;


Then your output will look like this:


PL/SQL procedure successfully completed.
Test



Reference: DBMS_OUTPUT (Oracle | Docs)

If you want to get rid of the PL/SQL procedure successfully completed message, then you can turn if off with the command SET FEEDBACK OFF. The whole thing will then look like this:

SET FEEDBACK OFF
SET SERVEROUTPUT ON
BEGIN
  DBMS_OUTPUT.PUT_LINE('Test');
END;


Your output will then be:


Test



Reference: How do I Suppress “PL/SQL procedure successfully completed” message in sqlplus? (Stack Overflow)
Reference: SQLPlus® User's Guide and Reference (Oracle | Help Center)*