This means your domain or subdomain is missing an A record. It is required for your domain or subdomain otherwise your website will not resolve. For example, if your website visitor tries to access your website like: yourdomain.org, it will not load. Also, if your MX record is missing an A record then you will not be able to receive emails.
How to fix:
Add an A record for your domain or subdomain which points to your web server's IP address or mail server (if MX record's A record is missing).