It may be that the device type you are using does not list SmartThings’ thermostat mode capability. It may also be that there are differences between the Sinope thermostat modes (Auto, Standby, Away, Manual, and X-Bypass) don’t match up with the SmartThings thermostat modes (Auto, Off, Heat, Emergency Heat, Cool). Here’s a screenshot of the SmartThings capabilities reference document:
I just posted a new DTH for the Sinope thermostats that includes presence, modes, and manual controls: [NO WORKY] Sinope Electric Baseboard Thermostat
If you use CoRE, you can the Sinope mode to Home or Away by turning on Expert mode in CoRE settings (which will cause the custom actions of presenceHome and presenceAway to be shown when you add a task). I will update the DTH with a custom action for setModestandby.