If you see this error:

Check your domain with DNSchecker and make sure that you have pointed it to the correct IP address.

If you are sure the domain has pointed correctly, try to uncheck the mail.yourdomain.com and re-issue certificate:

If you see Invalid response from http://domain.tld/.well-known/acme-... error it can mean that your domain is pointing elsewhere or you have some access-blocking rules in your .htaccess, this can be easily fixed by renaming (disabling) the .htaccess file.

If you see Error creating new order: acme: error code 429 "urn:ietf:params:acme:error:rateLimited" or Error creating new order :: too many failed authorizations recently: check https://letsencrypt.org/docs/rate-limits/ errors, as it means that too many attempts to install the certificate were made. In order to fix the matter, you will need to re-issue the certificate after 7 days.