I see this error in logstash-plain.log

Elastic Stack Elasticsearch
6

Have you set certificate params in output?

 elasticsearch {
    hosts => ["https://localhost:9200"]
    ssl_enabled => true
    ssl_verification_mode => "none"
    # ssl_certificate_authorities => ["/path/to/ca.crt"]
    # ssl_certificate => "/path/to/logstash.crt"
    # ssl_key => "/path/to/logstash.pkcs8.key"
}