That’s not the way this is going to work. You have to create the switch through the smartthings platform first. That’s what the instructions in the FAQ describe.
So far, it sounds like you are starting from the Alexa app. That is not what is described in the FAQ I gave you. So I am confused about which step-by-step instructions you are following. 
Why don’t you try instead creating a virtual switch using the following edge driver. Then make sure that this shows up in the smartthings app.
Once it is in the smartthings app, you should be able to discover it in the Alexa app, and then create an Alexa routine with it.
[ST Edge] Virtual Things Edge Driver
If you have any questions about using that edge driver, please ask in that thread, and someone there will help you.
Once you have the virtual switch created, and it shows up in your smartthings app, then you can go onto the next step of getting it discovered by Alexa.