XML validation Issue

I'm looking for a Freelancer to help me with the XML validation for my project. Specifically, I need the validation to be done using XML Schema. I'm validating a xml with the xsd that you find atttached. I'm getting the error "org.xml.sax.SAXParseException; lineNumber: 28; columnNumber: 76; src-resolve: Cannot resolve the name 'ext:UBLExtensions' to a(n) 'element declaration' component." The xml file supose to be well formed. The successful Freelancer will have extensive experience with XML Schema and be able to provide prompt and reliable service.