Ok I looked at the project file and I know why it doesn't work. You simply didn't tell the plugin what scripts to load. You have to put the name of the script to "Script files" property of a plugin in a Properties bar. You just added scripts to the project and the plugin has no way of knowing that.
I believe you didn't read the documentation. Do this: readymag.com/valerypopoff/valerypopoff-js-plugin
When you fix it, it works, but the canvas with the result is not visible and hidden under the game. If you go to the developer console and delete it, you finally get to see it. A small gray canvas the size of the button. It actually IS a button but without a caption on it:
But if you take a screenshot of the whole <body>, you'll see this:
Which is no surprise because this kinda tools are supposed to work with regular web-pages, not canvas-based games.
It was nothing. Just trying to call a function when 'All scripts were loaded' fired. Nothing else.
I just made this now since you asked. I was checking the console for errors. It comes up as html2canvas is undefined.