The namespace in the example is not valid.
https://docs.redhat.com/en/documentation/openshift_container_platform/4.18/html/virtualization/networking#creating-primary-udn-cli_virt-connecting-vm-to-primary-udn

In this example:
apiVersion: v1
kind: Namespace
metadata:
  name: udn_namespace
  labels:
    k8s.ovn.org/primary-user-defined-network: "" 1
# ...
The namespace has an underscore character in it, which is not valid in names. It should be changed to a dash.

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