Hi, You need to change the Canvas color. That is what shows when you use the eraser. Right click the image and choose Canvas Color. Then, change it to white. However, this changes the background for every capture.
It might be easier to make your selection with the selection tool, then make sure the Auto-Fill option is selected in the Tool Properties, and press Delete on the keyboard. In this way, what you want to erase is removed and the background is filled in for you at the same time.