Refer to the exhibits.

The FortiManager installation error is associated with the gateway value produced by metadata substitution, not with a general failure of FortiManager-to-FortiGate connectivity. A template uses a variable such as ' $(sdwan_port1_gw) ' to provide the interface gateway at installation time. If the current device has no valid value in its per-device metadata mapping, the generated FortiOS configuration contains an invalid IPv4 gateway and the copy or validation operation fails. Therefore the remediation is to inspect the variable definition and its per-device mapping, which is option C in the normalized question. The administrator can verify the mapping for the affected FortiGate, supply a valid address, regenerate the installation preview and reinstall. It is unnecessary to remove all template installation targets: FortiManager supports them alongside metadata variables. Nor must the members of a zone all use identical gateway configuration methods. This conclusion is specifically tied to the error shown in the original exhibit. The uploaded FortiSASE guides describe centralized management generally, but detailed per-device template substitution belongs to the FortiManager 7.6 Administration Guide.