The DNS record is there, but pointing to an address in private IP space. That is, your server is only accessible on your local network, and not the Internet at large. Look through the "How it Works" documentation page: Let's Encrypt needs to be able to get to your server (from multiple perspectives on the Internet, even) in order to validate that you own it, but if your site isn't publicly accessible then they can't.
You're very welcome. Note that if your site is intended to only be accessible on your local network, you can use the DNS challenge instead, but that requires being able to automate updating your DNS server (ideally) and of course the DNS server at least would need to be publicly accessible itself. But most people do intend for users on the Internet at large to be able to get to their site and find the HTTP challenge to be simplest, so for that the first step is making sure that your site actually is accessible to everyone.