"Get Started" button does not appear in welcome screen for FB Messenger bot
I found the solution. The "Get Started" button does not appear because I used the users who are not "Tester" for my app ( but the Welcome text appeared that why I confused)
There are a number of bugs/issue that seem to affect if the button appears or not once it is finally set using the POST request, this is what I have found.
You need to delete the conversion on facebook.com chat not messenger.com or mobile App. Click the little gear thing in the chat window to do this.
Once deleted on fb.com the button will appear the next time you try to chat on fb.com and on messenger.com
The button will not appear on any previously used Android Messenger clients after deleting on fb.com
To make the button appear again on Android Messenger client you need to clear the Messenger data/cache under Android Settings -> Apps , this will also clear your login and you need to login again.
I don't have an iOS device.