If you are trying to promote someone's product in Clickbank, there are chances that their url is banned in Google Adwords and that could be one reason you cannot promote their products.
The gist of the story is in 2012, you are not allowed to promote affiliate offers in Adwords or to an extent in Facebook as well and so Clickbank sites can only be promoted if you are a product owner and not by affiliates.
This does not mean you actually cannot promote Clickbank products but you have to do it in a round about manner.
You have to promote your sales page where you don't actually sale anything but grab users email address into your Autoresponder and then use the autoresponder to promote offers related to the list.
I hope this helps you earn from Clickbank