Your feed contains an element that this validator does not recognize in this context.
This may just be a typo. Element names are case-sensitive; make sure you're using the right case. For example,
pubDate has a capital "D".
This may simply be a case of an element being placed in the wrong context. For example:
itunes:category can only be placed inside of the channel element, it does not belong inside an item.
If it is an undefined
root element, then the document you are trying to validate is not a feed.
Verify that you have placed this element where the specification requires.
If, instead, you were trying to include a new element that you have defined, you will need to put it in a namespace.
Not clear? Disagree?
You might be able to find help in one of these fine resources.