Hi there. Unfortunately Nest has restricted their thermostats in SmartThings and does not allow them to be controlled through SmartApps when using the stock integration. If you would like to use Nest thermostats with RLA you can take a look at the MyNext Tstat integration which allows your Nest thermostat to work with SmartApps.
Alternatively you can use any standard ZigBee, Z-Wave thermostat or SmartThings supported WiFi thermostats and they will be able to work with RLA. We highly recommend the GoContol TBZ48L, Honeywell T6 and CT-100 z-wave thermostats for hosts since these have the ability to limit the maximum heating, minimum cooling temperatures and lock the controls remotely to limit what changes guests can make on the physical thermostats. Please refer to this topic for more details on the features available in these thermostats: [RELEASE] Enhanced Z-Wave Plus Thermostat Device Handler - Honeywell, GoControl, CT, Linear, Trane, MCO, Remotec