If it's a blank un-used DVD-RW, you would need to burn to it before you can erase from it.
There wouldn't be much point it erasing a blank DVD-RW if it's already blank. With DVD-RW's the first time you burn to it sets up the ISO header on the Cd, telling it what type of DVD it is and how it can be used.
If I've got that wrong then someone will correct me.