By default, Amazon has chosen to prevent the unlocking of doors. Since the Echo can respond to anyone’s voice, if you say left a window open someone could yell in to unlock the door.
As far a I have heard the only official lock that can unlock is the August through their Alexa app. But the invocation is not as natural as you must “ask August” and you must present an unlock code.
For SmartThings, Ask Alexa can be used which will work the same as how the August does above and offers a whole lot more.
You can also create a virtual switch that you toggle on/off and connect it with your lock through something like CoRE. Just remember that anyone who can talk to your Echo can then unlock (turn on/off) your door.