widget must have one includable

the basic tags for creating widgets are described in Page Element Tags for Layouts. If you just want to use the Page Elements tab to work with everything, then that's all you need to know. However, if you want more fine-grained control, this article describes what you can put inside a widget, if you're working in the "Expand Widget Templates" mode of the Edit HTML page.

The first thing to do is to add a closing tag. So this: