Let's Debug is a diagnostic tool/website to help figure out why you might not be able to issue a certificate for Let's Encrypt™.
Using a set of tests designed specifically for Let's Encrypt, it can identify a variety of issues, including: problems with basic DNS setup, problems with nameservers, rate limiting, networking issues, CA policy issues and common website misconfigurations.
Enter the domain and validation method you are having trouble issuing a certificate with. (Choose HTTP-01 if unsure).