Have a look here:
XML Extraction - Handling Namespaces (Tableau, Soap, OASIS types) - Help / Community - UiPath Community Forum
UPD1 - XNamespace Variable Usage
When ns is variable of DataType XNamespace and is holding the Namespace like:
ns = xmlDox.Root.Name.Namespace
then:
Descendants(“{ns}PlanDefinition”)
has to be changed to:
Descendants(ns + "PlanDefinition")