How can I enable the setting to toggle the Home Theatre Control setting to ON? It won't allow me to change it.

Apple TV tries to detect if it can find HDMI devices that have support for CEC. If none are found, or an HDMI error prevents CEC functionality, then the setting can’t be changed.

• Verify that the CEC function has been enabled on all HDMI devices in the setup. Not all device models have CEC support. Not all devices with CEC support include functionality for sleep-wake signals from other devices. Some AV receivers can be finicky for CEC. Some TVs were build to be the hub that controls peripherals, instead of listening to other devices. YMMV.
• Try disconnecting all HDMI cables, then reconnect. This will reset the HDMI connection(s), including CEC handshake. Cold restarting all HDMI devices would do the same.