
There is an unfilled rectangle over the top of the basketball background. This is weird because the select tool won't hit unfilled things unless you hover over the edge of them, and the edges are off the screen. But the fill tool hits that shape first and fills the whole screen. So you have this unremovable rectangle that's blocking the fill tool from working.
This behavior has been noted as weird during internal smoketests. If you draw an unfilled circle around your character, and try to use the fill tool on the character, it will only work on the circle. It's been suggested that the fill tool fill what's visible first, and hits "holes" last.