Hi, you need to go to the GitHub page for the software you are using (not Let's Encrypt, they didn't write this software) and ask for help there. They can point you to where to look for log files etc.
As mentioned on the other thread (Nextcloud letsencrypt cert unable to create - #5 by Osiris) you need to start by getting TCP port 80 and port 443 to forward to your server, that makes your web server accessible from the internet and in turn let's HTTP validation of your domain more likely to work.