Error: Unable to install kibana-plugin

Thanks for the details, I appreciate it. The docs for installing plugins mention running the command from your${KIBANA_HOME} directory, rather than from ${KIBANA_HOME}/bin.

Do you get the same error if you run this from /data/kibana-7.6.1-linux-x86_64 instead of /data/kibana-7.6.1-linux-x86_64/bin?