The most common culprit behind this error is Domain Name System (DNS) failure. FortiGate firewalls require a valid DNS configuration to resolve the hostnames of FortiGuard servers. If the firewall is configured to use internal DNS servers that are unreachable or misconfigured, or if the firewall itself lacks internet access, the query to Fortinet will fail. This is particularly common in "air-gapped" or isolated lab environments where the firewall has no path to the public internet.
If the list cannot be loaded but the administrator knows the DDNS IP or server name (for example, if using a 3rd party provider like No-IP, or a known FortiGuard server IP), it is possible to configure DDNS via CLI bypassing the GUI dropdown. The most common culprit behind this error is