Your thermostat has a lock feature on it preventing manual changes from being made on the thermostat itself.  (Please note, even if the thermostat is locked, changes can still be made through the website or mobile app).

To both lock and unlock your thermostat, push the two silver up/down arrows at the same time.  When the thermostat is locked, you will see a small key appear on the right hand side of the thermostat screen.  When the thermostat is unlocked, the key will disappear.