... the space you are
trying to fill is not a polygon. It looks it's just the empty space
showing through. Try outlining it with the Draw Polygon tool.
A quick way:
Draw a rectangle that covers the whole area. Select the rectangle and
the driveway. You may need to orbit to where you can lasso them both.
Right click and do Intersect Selected. That will cut the rectangle
along the line of the driveway. Then you can fill in the grass and
delete the part of the rectangle that overlaps the driveway.
You might want to do the same kind of thing where you have two
sections of driveway overlapping.