I created simple node.js application (source code from here https://azure.microsoft.com/en-us/blog/visual-studio-code-and-azure-app-service-a-perfect-fit/)
I believe that you need ${workspaceRoot}/server.js, not ${workspaceRoot}/app.js for program. The code you're using doesn't have an app.js, that's what that (poorly worded) error is telling you.