Eraser tool can be activated through the main menu -> Paint -> Eraser -> ...
Or you click at the following button on the Painting Tools panel:
(you can open this panel through the main menu -> Paint -> Show Paint Tools).
You can adjust the size of eraser, and select its type:
1.Standard eraser. It allows you to erase all graphic under the cursor. You can define the feathering value (in pixels) for the standard eraser.
2.Background eraser. This is AI-based tool. With it, you can only erase the background behind an object if the object contrasts enough with the background. You can set the tolerance (%) value here.
Warning! The image layer must be selected in the layers list in order to use eraser.