The following thread is the most up-to-date on the official SmartThings/echo integration, so I would ask your question there.
That said, two quick thoughts.
One) you might want to just use the IFT TT integration between echo and harmony to begin with while you’re figuring everything else out. It’s very easy, does not require any virtual switches, and actually doesn’t use smartthings at all. It does use the “Alexa, trigger Disney Channel” format, and some people don’t like the use of the word trigger. So you may well want to replace it with the official smartthings integration once you work that out. But meanwhile, you’ll have immediate successful voice control of harmony through echo, which is fun. And you get to start working out the quirks of harmony activities, which is going to be essential. Before you can do anything else, you have to have an activity set up in harmony that will do what you wanted to do.
Just look at the first section in the following thread for how to use the IFTTT trigger method. Don’t look at anything else there. The rest of it is all out of date.
- The names of your switches should just be simple names. Don’t use “please” or “turn on” or anything like that for your virtual switches. You’re going to be saying “Alexa, turn on {name of your switch}.” You don’t want to be saying “Alexa, turn on please turn on the television.” Your switch names should be as short and distinctive as possible, typically one or two words. So “television” “DVD” etc. it may help to think of these as the button names on the TV remote. When you want to tell Someone else to press a button, which button name would you use? You probably wouldn’t name a button “please turn on the fire TV.” The button name would just be “fire TV.”
You will find a few short names that won’t work because echo has reserve those for their own use. This will include things like play, pause, power, off, Etc. so you have to think of something else to call those. see:
If you’re using the official method, it now creates a virtual switches for your existing harmony activities at the time that you add the harmony hub to smartthings. The last time I looked at it it also appended [harmony] to each switch name, which most people remove since it just gets in the way when you’re talking to echo.
Anyway, make the switch names short and unique. Just putting a please in front of everyone is going to cause echo to give you that response “which device did you mean?”
Hang in there, because once you have this working it’s both very practical and lots of fun. 
That’s one reason I recommend starting with the IFTTT method for a few activities while you’re working out the set up for the other method just so you can get the inspiration of having something working right away.
Again, though, look at the first link I posted for the most up to date information on the official integration. 