Your domain (and its associated aliases) must resolve to your server before you enable Let's Encrypt.
- If your DNS records are set up correctly, you may need to wait a few minutes before it propagates. You can always add your domain and enable SSL later.
- If you are still getting this error, remove all aliases associated with your domain before trying again. Then, add back the aliases one by one - each alias needs its own certificate, since enabling SSL for a domain will also enable it for all of its aliases.