Missing namespace for
The spec states that "[an] RSS feed may contain elements not described on this page, only if those elements are defined in a namespace.".
As this element may not be used in RSS 2.0 feeds, you will need to find another element in a namespace that has the same meaning, or convert to a feed format which does not contain this restriction.
Not clear? Disagree?
You might be able to find help in one of these fine resources.