Gimp: eraser gives me brush instead of erasing?

That's easy to fix, you need to right click the layer in the layers panel and choose "add alpha channel".

When a layer has no alpha channel, the eraser works as a brush and will paint with the background color to replace the foreground color. Only when the alpha channel is enabled on a layer will the eraser erase to transparency. If you don't want to erase to trasparency the eraser will use the background color to paint with. You can change the background color by clicking on the background color square to change it so it matches the background colour of your image.

Double check what color the eraser is, sometimes it'll just be a paintbrush if it's set as the wrong color. If that's not it, save your project, uninstall the program and reinstall it