Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> set serverout on
SQL> DECLARE
  2    v_i NUMBER;
  3  BEGIN
  4    SELECT Nvl2(1,1,1) INTO v_i FROM dual;
  5    Dbms_Output.put_line('Result :' || v_i);
  6  END;
  7  /
Result :1

PL/SQL procedure successfully completed.

SQL>
SQL>
SQL> BEGIN
  2    Dbms_Output.put_line('Result :' || Nvl2(1,1,1));
  3  END;
  4  /
  Dbms_Output.put_line('Result :' || Nvl2(1,1,1));
                                     *
ERROR at line 2:
ORA-06550: line 2, column 38:
PLS-00201: identifier 'NVL2' must be declared
ORA-06550: line 2, column 3:
PL/SQL: Statement ignored

select NVL2(supplier_city, 'Completed', 'n/a')
from suppliers;