Hi @jaemin_park, welcome to the community!
I am sorry you are having trouble. This message is thrown when the n8n server can’t find a connection for its SSE messages. If this happens only occasionally a simple page reload will typically fix the problem.
Seeing you are using K8s I suspect you have additional services between n8n and your browser which might need additional configuration here. Nginx for example would require configuration similar to the one discussed here in order to proxy such messages.
When using external services in your routing (such as Cloudflare) there is also a good chance these might cause additional problems depending on their setup. So you would want to ensure these don’t cache anything or otherwise interrupt/reset connections.
Lastly it might be worth checking if websockets work better for you. These will replace SSE in v1 of n8n, but you can already use them by setting the N8N_PUSH_BACKEND=websocket environment variable.