Hey @Keith_Li, you need to have at least 1x AP with 6 GHz radio connected to the Network in order for 6 GHz settings to show up in SSID/Radio Settings pages, I'd also ensure you are on the latest Stable firmware release.
If you still don't see the settings it may be that Meraki haven't been able to enable the support in your region yet, I'd reach out to support for confirmation.