Error attribute URL

I want to create a product feed for Google Dynamic Remarketing with the plugin Product Feed Pro. When I fill in the field mapping and after download the XML file, I get this error:

error on line 2 at column 100: Specification mandates value for attribute URL

I don’t understand why it can’t pull the URL, because I also have a custom feed where it does not have a problem pulling the URL. What can I do to fix this? Thanks in advance.