This happens when the widget area of a page's layout no longer exists. Adding these widget areas back will allow widget drag and drop. In this article we will be fixing the product details page.
Open your active theme: Themes > [Theme Name]
From the left menu expand Pages > Product Details Then select HTML Editor
As you can see in the screen shot of the HTML Edit rows 12 and 14 are missing the code to display the widget.
You will need to click into the one of these rows and add the following code :
<ac:widgetarea id="ProductRightTop">
</ac:widgetarea>
The id is just an arbitrary name and can be named whatever you want.
Once this has been added click save in the upper right hand corner of the theme manager. Now click the “Widgets” link in left hand menu. You will now be able to drag drop a widget in to the Product Right Top area.
