I don't know where you can use \imagex, i am on 15063 and ADK for 15063 and use the dism cmd window from the kit. If you have imagex somewhere else then you have to use the full path to it.
imagex does work if you have ADK installed, while if like myself uses the one from e.g. abbodi1406's ESD > ISO then you can execute it within an elevated powershell using .\imagex