Missing namespace for foo

This element is in a namespace, but you have not declared the namespace.

Declare the namespace at the top of your feed. For example, if you use dc:date in your feed, you will need to declare the proper dc namespace, like this:

<rss version="2.0" xmlns:dc="http://purl.org/dc/elements/1.1/">

Check the full list of namespaces for the exact declaration you need.