There is sadly no eraser :( BUT you can use the white color to erase from the right side of the color picker. If your board if white then you pick white, if you make a mistake then you can click the "undo" button on the left.
The only way to erase is to pick a color using the color picker and cover over it, or click the undo button, there are no eraser on Drawingnow D: but I hope that helps :)!