This looks like a connectivity error between your ACME client and the Let's Encrypt API. Can you verify that your server is able to successfully connect to other external sites?
Can you share the output from these commands (run on your server):
curl http://ipv4.whatismyip.akamai.com/ ; echo
curl http://ipv6.whatismyip.akamai.com/ ; echo
dig +short whoami.ipv4.akahelp.net TXT
dig +short whoami.ipv6.akahelp.net TXT
dig +short whoami.ds.akahelp.net TXT
dig +short whoami.ds.akahelp.net TXT
dig +short whoami.ds.akahelp.net TXT
mtr -c 20 -w -r acme-v02.api.letsencrypt.org
You may need to install a package to get the mtr command (I believe its mtr-tiny on Ubuntu/debian systems)