Let’s Encrypt is popular free SSL service. My VPS which is using Virtualmin was unable to use Let’s Encrypt and throw an error “ACME v1 disabled”. To fix this problem, execute the following line in terminal:
apt-get install socat certbot certbot register
After that, complete the register with a valid e-mail address and try to request new Let’s Encrypt certificate via Server Configuration – SSL Certificate – Let’s Encrypt menu.