It is because Let’s Encrypt has currently disabled the TLS-SNI-01 challenge due to an identified security issue.
The official has provided an workaroud at the Let’s Encrypt community website as following:
If you’re serving files for that domain out of a directory on that server, you can run the following command:
sudo certbot --authenticator webroot --webroot-path <path to served directory> --installer nginx -d <domain>
If you’re not serving files out of a directory on the server, you can temporarily stop your server while you obtain the certificate and restart it after Certbot has obtained the certificate. This would look like:
sudo certbot --authenticator standalone --installer nginx -d <domain> --pre-hook "service nginx stop" --post-hook "service nginx start"
2018-01-19 update
Let’s Encrypt just released Certbot 0.21.0, which use HTTP-01 challenge type instead of the compromised TLS-SNI-01 by default for apache httpd and nginx. Update your certbot to get rid of this issue.