Techically, that's an arc not a quarter circle. It might seem trivial, but an arc, as your error suggests, is not bound. A quarter circle would have an arc joined by two lines that are perpendicular to each other (like a slice of cake) - that's a bound shape. An arc is just a line and you can't fill a line.
If you are filling both blank areas with the same colour, I would draw a box the same shape, fill it, and put it behind the arc.
If you want to fill each blank area with two different colours, I would do the same again, but then I would use

tool to close the path (i.e make it a quarter circle) and fill it with the other colour. Note: to use

to add to the path, select the path then start drawing from one of the end nodes.
Note, if your using

you don't need to. Just set the fill property for the selected object. The

tool creates new objects in the shape of the area you are using it on... I hope that makes sense.