I ran this command, as root:
certbot certonly --webroot -w /srv/http/nginx/ -d caver.ddns.net
It produced this output:
An unexpected error occurred:
There were too many requests of a given type :: Error creating new cert :: Too many certificates already issued for: ddns.net
Please see the logfiles in /var/log/letsencrypt for more details.
The replies to that issue are still accurate. You’ll have to get your own domain (dot.tk for a free option) or switch to a dynamic DNS provider that’s on the Public Suffix List.