check out this thread: Encrypting communications in Kibana
in short: Due to clients(Web Browsers) not trusting self-signed Kibana certificates, you will see a message similar to the following in your Kibana logs, until proper trust is established by using certificates generated by an enterprise or public CA (here's the link to the issue in the Kibana repo). This issue does not affect your ability to work in Kibana:
[18:22:31.675] [error][client][connection] Error: 4443837888:error:14094416:SSL routines:ssl3_read_bytes:sslv3 alert certificate unknown:../deps/openssl/openssl/ssl/s3_pkt.c:1498:SSL alert number 46
As a result, Kibana's error logs is normal.