When you've been erasing something in your work, you might have encountered, that you've been accidentally erasing part of
the canvas too. That's why it's important to work non-destructively, by working across multiple layers.
Navigate to the right-hand side, until you see layers, you should find there "Layer 1",
which is your background layer.
With the plus icon, you can add new layers, and all layers can be renamed to manage your layers more easily.
After creating an new layer, make sure to select it first, and then proceed to make changes to your image.
Once you feel like erasing some changes you did, just pick up the eraser tool and remove them,
and you'll notice your background being untouched.