As Ubuntu 14.04 itself stopped receiving support and updates last April, it’s no longer supported by the Certbot project.
There aren’t any versions of Certbot available for Ubuntu 14.04 which support ACME v2. You would need to upgrade to Ubuntu 16.04 LTS (or newer) to keep using Certbot.
Alternatively you can look into other clients like acme.sh, but you will need to re-do your configuration in that case.
Your current certificates will continue to renew until June 2021 (source), so if I were you, I would start slowly working on upgrading to a supported version of Ubuntu.