I am getting the error "Directory field not found': new_authz" when i run the command "certbot certonly --agree-tos --webroot -w /var/www/acme-challenge -d myapp.com --server https://acme-v02.api.letsencrypt.org/directory".
I am on Ubuntu 14.04.4 LTS (GNU/Linux 4.4.0-148-generic x86_64) and using certbot 0.15.0
Your certbot is too old to be compatible with ACME v2. (It's not enough to just point to the v2 endpoint, it actually needs to speak the v2 protocol.)
See this thread for more info on how to update:
Where did you manage to get thát? 
It's ancient!
You might want to run ubuntu-security-status to see if you are still getting security updates for your OS (in my understanding, you would have to be paying for a subscription in order to continue to get them at this point in time).
Unfortunately, as the Certbot team discovered in the past, OS packagers don't necessarily consider upstream compatibility with a new API (like in the ACMEv1 → ACMEv2 switch) to be a "security update", so you would be somewhat likely never to get an ACMEv2-capable update from the OS package manager at all!
Also unfortunately, I believe there is no officially-supported way to install the current version of Certbot on Ubuntu 14.04 LTS (maybe @_az could confirm this for us?). In that case, you might need to switch operating systems or ACME clients.
If you do have Python 3.6 available
https://certbot.eff.org/docs/install.html#system-requirements
then you could try one of the alternate installation methods for Certbot in the hope of getting an up-to-date, ACMEv2-capable version.
https://certbot.eff.org/docs/install.html#alternate-installation-methods
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