Kibana server is not ready yet, error : Unable to get issuer certificate

Elastic Stack Kibana
2

Your kibana is configured to trust

elasticsearch.ssl.certificateAuthorities: [ "/etc/kibana/certs/elasticsearch-ca.pem" ]

Is this the CA that is used to sign the following on the ES side

xpack.security.http.ssl:
  enabled: true
  keystore.path: /etc/elasticsearch/certs/http.p12

You said

I change the certs for the ones from my company.

So it is possible that you HTTP cert is not signed by the above CA and hence not trusted by Kibana.