Got PLS-00306 error but don't know why

Hello there,

I have a simple script and I got PLS-00306 error.  I have no clue why.  Here is the script:

Begin

For xx in (select vendor_id, vendor_name from ap.ap_suppliers)

loop

    DBMS_OUTPUT.PUT_LINE (

         xx.vendor_id,

         xx.vendor_name);

END LOOP;

END;

If I use only one column, anyone, I don't have this error.  Please help.

Thanks,

Ning