If you’re using a 5.x version of Blender, they recently changed how strokes/fills work. If you go to edit mode, select the strokes you want to be fill, then under Stroke -> Set Stroke Type, you can chose stroke, fill, or both. In 5.2, there’s a set of buttons between Strength and the end cap buttons where you can set this in draw mode for the next stroke you want to do.
