Well, it might work from where you are, but it doesn't work from much of the world. Here's one site I often use to check connectivity from different places:
Let's Encrypt checks from many places around the world, so it's saying that some places (its "primary" validation) could connect, but others (the "secondary" validation) could not.
You might find this FAQ helpful:
If you're not expecting connectivity to be different from different places in the world, you might need to ask your internet hosting provider about it.
Side note: Ubuntu 18 is out of support (unless you have a support subscription), and Certbot 0.27 is really old too. You might want to update if you're actually caring about security, and not just caring about getting a certificate. 