It produced this output:
Saving debug log to /var/log/letsencrypt/letsencrypt.log
Plugins selected: Authenticator standalone, Installer None
Obtaining a new certificate
Performing the following challenges:
http-01 challenge for herdtrails.com
http-01 challenge for www.herdtrails.com
Waiting for verification…
Cleaning up challenges
Failed authorization procedure. www.herdtrails.com (http-01): urn:ietf:params:acme:error:dns :: DNS problem: NXDOMAIN looking up A for www.herdtrails.com, herdtrails.com (http-01): urn:ietf:params:acme:error:unknownHost :: The server could not resolve a domain name :: No valid IP addresses found for herdtrails.com
To fix these errors, please make sure that your domain name was
entered correctly and the DNS A/AAAA record(s) for that domain
contain(s) the right IP address.
[root@me ec2-user]# sudo -H /opt/letsencrypt/letsencrypt-auto certonly --standalone -d herdtrails.com
Saving debug log to /var/log/letsencrypt/letsencrypt.log
Plugins selected: Authenticator standalone, Installer None
Obtaining a new certificate
Performing the following challenges:
http-01 challenge for herdtrails.com
Cleaning up challenges
Problem binding to port 80: Could not bind to IPv4 or IPv6.
does it mean that is the same problem as before? in that case I have no idea how to proceed.
If you have a running webserver, why do you use standalone? Standalone starts an own webserver, but then port 80 must be free. But your webserver blocks port 80.