Editing and replacing PDF XMP Metadata File

I'm trying to edit a PDF XMP metadata file, but I am receiving an "Unknown Error" popup when I try to load the XMP file back into the PDF. I'm saving the PDF metadata XMP file, opening it and adding additional metadata fields to it in an XMP/XML editor, and then uploading it back into the PDF file. It works when I add one element (field). But, the error occurs when I try to upload an XMP file where I create a Child element as a part of the scheme. Does anyone know why I am getting the "Unknown error"? Can the PDF handle Child elements for some of metadata elements?