That's because there isn't a feature or setting to do that unfortunately. You can manually set the thermostat to "off mode" but there isn't a way to schedule that. Are you looking for your HVAC system to be fully powered down, or just taken out of heating or cooling mode in off hours? If you're talking about an actual powering down, that is beyond the scope of the Nest Thermostat capabilities. Do you just not want it to come on outside of schedules? Is there a specific range you're concerned about?
If you forget the thermostat for a minute, what is it you want your HVAC equipment to do during the times the thermostat is off? Maybe there's a toggle switch on your furnace/HVAC equipment you can use? As an example, I've seen other users replace those with Kasa smart switches (or similar) to be able to schedule off and on. If this applies to you and you decide to do it, just be aware that with your HVAC system powered down entirely, the thermostat can't engage the safety temperatures feature if things get too cold to turn on heating to prevent pipe freezing, or to turn on the cooling to ensure your home won't overheat.
Nest thermostat Off mode - Google Nest Help
Thermostat Safety Temperatures - Google Nest Help
