I hate explaining this because most people won't understand.
Okay, the PHP script uses the points system. It checks to verify you have enough points when you click the link to "buy glowing username." Because of this, if you don't have enough points you will get the error. The coding is poor and it doesn't care if it's free or not -- it's just doing what the PHP code tells it, which is to reject people with less points than the actual item cost.