After you paste a screenshot in Paint.NET, a selection is visible around the edge of the screenshot. The selection indicates that any edits only available in the area of selection. This even applies to ms paint, photoshop.
Solution:
To draw/write/paint outside of the pasted screenshot, simply deselect by pressing CtrlD or click the menu Edit -->> Deselect
. Now you will be able to edit outside of the image.