Anyway, one way to get Echo to unlock A door when the lock is controlled by smart things is to set up a routine that does the unlock, then create a virtual switch that triggers that routine. Now you can have echo turn on that switch.
Another popular alternative is just to use IFTTT. In that case you don’t need a virtual switch at all, you just use the echo service/channel for the If and the SmartThings service/channel for that.
The clunky part is choosing what you want to say. Since it’s a switch, echo expects you to say “on” instead of “unlock.” Some people are fine with saying “Echo, turn on the front door.” But most people don’t like that.
So that’s where you have to start getting creative. 
If you use IFTTT, you can use any phrase you want, but you have to put “trigger” in front of it. So at my house I just say “echo, trigger unlock the front door.”
If you really truly just want to say “Echo, unlock the front door” you can, but you will have to use one of the advanced Echo management smartapps like EchoSistant or AskAlexa which have been developed by this community. They work great, but the set up is pretty complicated. There are lots of people who will help you do that, but it’s just something to be aware of.
You can find both of those advanced options in the quick browse lists in the community – created wiki in the project report section on the “voice” list.
The videos in post one of this thread are plain-vanilla descriptions of the standard features currently provided by Echo. But you can do way more than that if you have SmartThings. This post describes four different ways to have SmartThings enable echo to unlock a lock. ( virtual switch plus routine, IFTTT, EchoSistant , askAlexa) But there are even more than that. Some are easier to set up than others, some offer more choices of verbal phrase, but the point is it can definitely be done. And for some of us, that’s a very important fact. 