Hi Colleen,
This could be happening for a couple of reasons:
1. If you are using the form on a landing page, you need to add the name of the landing page as opposed to the URL in the Webpage: constraint.
2. If you have hardcoded the form on a landing page, Marketo cannot recognize that there is a form on a landing page and thus giving you an error when you input the URL. This second option is less common.
Let me know if that works for you,
Raul