Error message "entry door sensor open"

A sensor which is open when the system is armed is ignored until it is closed, then it will set off an alarm if opened.

If you have that message on your keypad display, and if you're certain the sensors are closed, just hit the little x on the icon for the message to clear it.  Arm in Home mode, see what it does.  Then disarm it, see what it does.  Hopefully, just clearing the message fixed it - but if not, you might still have an open sensor somewhere - If you named your sensors (SS3), the keypad display will tell you which one it is - sometimes the sensor and magnet are too far apart or too close together, may require slight fiddling with it, sometimes all you have to do is move the base station closer to the sensor (often only a couple inches is all it takes).  

Lastly, open/close that door or window with that sensor, SLOWLY, watch the sensor to make sure the blue led light blinks (once for opening, twice for closing) - check keypad display for no errors, test in Home mode again, disarm again.  If none of this resolves the issue, call SS for troubleshooting/possible replacement.