On my Smartcode lever, the Lock button does not lock the door.

If there are no codes in the lock, the lock button will not lock the door. This is indicated by the keypad flashing red three times with three beeps. Program at least one user code and test the Lock button again.
The battery is too low to lock the door.
This is indicated by the keypad flashing red three times with three beeps. Remove the batteries and replace with a fresh set.
The set screw in the lever was not fully tightened.
Fully tighten the pre-installed set screw at the bottom of the interior lever.