That screen expects data to be passed to it. That data must be sent as part of the message in this case, inside flow_action_payload. See https://developers.facebook.com/docs/whatsapp/flows/guides/sendingaflow

It'll be something like:

...
flow_action_payload: {
 screen: 'ADDRESS_SELECTION',
 data: { countries: <the content of the __example__ field for countries>, ... }
}