lets try this

system-apps/guestbook/guestbook-ui-deployment.yaml

@@ -2,6 +2,7 @@ apiVersion: apps/v1
 kind: Deployment
 metadata:
   name: guestbook-ui
+  namespace: guestbook-ui
 spec:
   replicas: 1
   revisionHistoryLimit: 3

system-apps/guestbook/guestbook-ui-svc.yaml

@@ -2,6 +2,7 @@ apiVersion: v1
 kind: Service
 metadata:
   name: guestbook-ui
+  namespace: guestbook-ui
 spec:
   ports:
   - port: 80