This error occurs when you attempt to open a project as a website. The easiest way to determine if you've created a website or a project is to check your solution folder (i.e. where you saved your code) and see if you have a *.sln file in the root directory, if you do then you've created a project.
Just to add, I encountered this error just now when I attempted to open a project I created a while back by selecting "File", "Open Website" from the Visual Studio menus whereas I should have selected "File", "Open Project" instead.
Hope it helps!!