I installed logstash 6.4 and I am seeing this error:
It seems your ruby installation is missing psych (for YAML output). To eliminate this warning, please install libyaml and reinstall your ruby.
I installed libyaml and I didn't even have ruby installed, so I installed that, but logstash still threw these errors after installation of ruby and libyaml. I also tried reinstalling logstash after installing both of these, but still have the same error.