You are trying to create an XML v2 with XPath rules file-type filter but after adding your .xml file in order to define the parser settings, you receive the following error message: 'xx' is an undeclared prefix. Line xxxx, position x

The issue is caused by undeclared namespaces present in your source document(s). Open your file(s) in a text editor and check its syntax. You should receive a set of parsing errors that point to the lines on which the undefined namespace prefixes are found.

Remove all of the undefined namespaces or define them in your XML header, save the file and then try to re-create your filter.