instructions install letsencrypt with acme.sh on ubuntu 16.04

i need instructions (no acces to server i need learn how it works) on how to install ssl certificates
an my servers

system
ubuntu 16.04.3 LTS
apache2

i already installed acme.sh
i use tomcat for java websites located like etc/tomcat/webapps/website
i use libapache2-mod-jk to run java websites with apache

i tried issue a certificate but get error “unable to load Prviate key”
see attached screenshot

i have servers where previous freelancer has made everything running.
so it works somehow
i am not interested in suggestions i should use certbot that will do all for me
i want make all settings myself with acme.sh and apache2