The fill tool is meant to replace smaller selections, and even then it doesn't always get it right.
With such a large selection, it has no idea what you are trying to cut out and what it needs to replace it with.
I was thinking that maybe the grass was just too complex for it, ive seen vids online where it takes much the same size or larger area than i have there without this issue..
What you need to do is to use the marquee tool to create a selection of the bottom part of your picture, then copy the selection onto a new layer (Command J) and then (command T) to transform the new layer by stretching it to fill. It usually works for this kind of fill.
Not tried that way before I'll have to research that, I was just wondering if I'd missed a step or got a setting out, as I've seen video tutorials that did the select pixels, inverse, then fill tool, I've got the expand setting on 30 pixels maybe that needs to be more or less?