Is it possible to set up letsencypt on a LAMP stack with openvpn already installed and running?

When I run “sudo certbot --apache -d mydomain.com” the response I get is:

The following errors were reported by the server:

Domain: mydomain.com Type: connection Detail: Error getting validation data

To fix these errors, please make sure that your domain name was entered correctly and the DNS A record(s) for that domain contain(s) the right IP address. Additionally, please check that your computer has a publicly routable IP address and that no firewalls are preventing the server from communicating with the client. If you’re using the webroot plugin, you should also verify that you are serving files from the webroot path you provided.

Thanks very much, RD