Troubleshooting Tip: How to fix error '(errcode)-999 - channel X is not supported' in FortiManager 7.4

The error:

This issue appears after upgrading FortiGate to 7.4 release where:

• FortiManager is managing 7.0 or 7.2 ADOMs.
• FortiGates are managing FortiAPs.

Apply the following workarounds to fix the error: '(errcode)-999 - channel X is not supported':

1. Disable central management and create new AP profiles/templates to be assigned to the 7.4 FortiGates and its FortiAPs.
2. Use a CLI/Jinja template to override the band configuration.

The ADOM version and FortiGate version must be the same to solve this error permanently.