Well, the obvious answer is that it can't find the file. Either it's looking in the wrong place, or the web server doesn't have permissions to read it.
By the way, this isn't about your specific question, but the way you're using your files looks very strange. Why do you have all the HTML document stuff (<head> and <body> tags etc) in all three files? It should only be in the template. The PHP file shouldn't have any static content - it's just passing things on to the template engine. And the welcome.html file is being included in the middle of a page. If it was working, you'd be getting output something like this: