Talking Thermostat also saying "two sixty four"
I don't know what that error code means, but you can reset this unit by pressing and holding both the "Reset" button and the "No" button simultaneously for at least ten seconds, then pressing the "Run" button.  You'll have to reprogram the thermostat.  

My next step would be to replace this with a standard programmable thermostat, the talking thermostat is mostly a marketing gimmick, it was promoted to HVAC contractors as a device that would generate calls for routine service and maintenance.