There are many different ways to accomplish this, but you need to pick just one method and use that all the way through. It sounds like you are mixing parts of different methods and that’s why it isn’t working.
The first step for any method is to create a harmony activity that does exactly what you want whether Echo is involved or not. So make sure the “watch tv” activity exists and works exactly as you want it to when you select it from the Harmony remote or the harmony app.
Once you have that, decide which method you want to use to include echo as the starting method for that harmony activity. Then follow the exact steps for that one method and it should work. If you have any questions, ask in the specific thread for that method and people will be glad to help you. Do not mix together pieces from multiple methods. Just pick one method.
After you have one method working, if for some reason you want to try one of the others, that’s fine. Just pick a second method and follow all of the steps for it all the way through. And so on.
Here are the methods. Again, pick only one and follow all of the steps for it all the way through. If you run into a problem, don’t shift to a different method – – ask for help on the thread for that method.
Because different people use different methods, no one will be able to help you with a mix of everything, because no one uses everything. 
Before you do anything, you must’ve created a harmony activity called “watch TV” that works the way you want it to. This is true for all of the following methods.
- Yonomi. Yonomi is a free app that works with both harmony and echo. It does not work with smartthings so smartthings will not come into the picture. To use it, follow the steps at Yonomi. Follow only those steps, don’t try to mix in anything from smartthings at the same time. The end result is that you will be able to say “Alexa, turn on watch TV.” And echo will trigger your harmony activity called “watch TV.”
Here’s the community thread, but you probably won’t need it – – it’s a very easy app to use. Just make sure you’ve created the harmony “watch TV” activity first.
- IFTTT. IFTTT is a free service. If a manufacturer has a “channel” there, you can connect events on one channel with events on another through a “recipe”. This works very well.
Echo, harmony, and SmartThings all have IFTTT channels. For this method, again, we are going to ignore smartthings completely. We are just going to tie and event in the echo channel directly to an event in the harmony channel. So you have to have authorized both your Amazon account and your harmony account to IFTTT.
At that point you can define a “trigger phrase” in echo channel. Yours will be “Alexa, trigger watch TV” that will be the “if” in your recipe.
You will then select from the harmony channel (not the smart things channel) and choose the “watch tv” activity as the “that.”
The end result is that you will be able to say "Alexa, trigger watch TV " and echo will trigger your harmony activity “watch TV.”
Note that saying the word “trigger” is required for this method. Echo’s response will be “sending that to IFTTT.”
The detailed instructions are in the first post of the following thread. Ignore the rest of the thread, it has to do with other methods.
- SmartThings. There is an official SmartThings/Harmony integration and and an official smart things/echo integration. This has changed many times, which is probably why you got confused. To use this method, follow the steps in the following thread, and only that thread. Do not mix it with steps from any other thread. You will not need IFTTT.
If you have any follow-up questions on that method, ask them that thread. And only that thread.
The end result is that you will be able to say “Alexa, turn on watch TV” and (pay attention, because this is different from the other methods) Alexa will tell smartthings to turn on a virtual switch called “watch TV” and turning on that virtual switch will cause smart things to tell Harmony to turn on your “watch TV” activity.
( Although the virtual switch and the harmony activity have the same name in this example, they don’t have to. The virtual switch could be named “banana” and all you would say “Alexa turn on banana” and the end result would still be the “watch TV” harmony activity coming on. )
If you want to use this method, follow the instructions in the following thread and only those steps. Ask follow-up questions about this method in that thread. I personally don’t use this method and can’t help you with it.
(Note that this thread says “2016 version”. That’s very important. Most of the other threads discuss the 2015 version of the official integration, and it was quite different. So if you want to use the official SmartThings/harmony integration with Alexa, this is the thread you want.)
- Ask Alexa skill. this method is for programmers. It’s very powerful, but it’s only going to confuse non-technical people. If you don’t know what a raspberry pi is and you don’t know how to set up your own server and you don’t know what AWS stands for and you don’t have an AWS developer account, skip this one. At least until you have one of the other three working well for you.
Very technical people can use this method to get the echo to do things that it is not designed to do out of the box, like answer questions about whether the door is locked, using the official “Alexa skill” method from Amazon. But it’s not easy to set up and until you have one of the other three methods working, it’s not going to make any sense to you at all. So very powerful, but most people and save it until you have one of the other methods working first.
If you see people in the forum talking about “ask Alexa” they’re probably talking about this method, but ask to be sure.
The command format for this method is “Alexa, ask smartthings to watch TV” where the “ask smartthings” is a required part of each phrase in order to access the skill.
- Official Harmony/Echo integration there is a new official direct integration that’s in beta for echo and harmony. You have to be in that beta program to use it. But people are starting to talk about it so I just mention it here. If you hear people talking about the “harmony beta,” that’s probably the one they mean.
Summary
So that’s it. Pick one of the first three methods and get that working. Follow the steps for that method and only that method. Ask follow-up questions in the thread associated with that method.