Thats okies in the staff_items.php
look for these line..
usually you will get the error message if a POST isnt being sent to the INSERT or UPDATE
example
if(!isset($_POST['itmname']) || !isset($_POST['itmdesc']) || !isset($_POST['itmtype']) || !isset($_POST['itmbuyprice']) || !isset($_POST['itmsellprice']))
{
print "You missed one or more of the fields. Please go back and try again.<br />
<a href='admin.php?action=newitem'>> Back</a>";
$h->endpage();
exit;
}