That's hard to say without more details, such as log files and more information about your environment and how you're requesting the certificate. If you were using AutoSSL, this might be a good place to get started.
Part of the process is validating the domain ownership by (typically) sending a HTTP request to the domain, requesting a file under
/.well-known/acme-challenge and verifying that the response is as expected. If that domain is currently not available or causes a timeout, that might explain the issue you're having. The validation request is sent from a Let's Encrypt server, so the website needs to be publicly accessible.
It's also possible that there was some kind of outage or maintenance window when you requested the certificate; these happen from time to time and are announced at https://letsencrypt.status.io/. In any case, if you haven't tried again since you ran into that error, that might be worth doing.