Are you connecting to Elasticsearch over https? If so, you need to make sure that certificate is trusted by Kibana. If a custom CA was used for that certificate, then you'll need to set elasticsearch.ssl.certificateAuthorities: [ "/path/to/your/CA.pem" ] as well.
If not, then I have a few more questions:
When is this error appearing in the logs? Are there any other errors?
Does Kibana function despite these errors, either partially or fully?
Is Elasticsearch configured to require a client certificate?
Thanks for posting. Nothing else seems out of the ordinary to me here.
Is your client (browser) also configured to send a client certificate? We've seen similar errors in the past when the browser tries to send a client certificate, but Kibana isn't expecting one. Which browser are you using to connect to Kibana?
@Yungyoung_Ok you can definitely use the certificates created by elasticsearch-certutil in Kibana. In my testing, I'm seeing this error logged when connecting using a certificate that the browser doesn't trust, and you proceed anyway with the warnings. In this situation, that log message can be ignored, I've created https://github.com/elastic/kibana/issues/35004 to improve the logging experience in these situations.