The eraser from the Local Adjustments sub-palette is inactive. The eraser appears with a red barred circle.

This information applies to version(s): 1, 2, 3, 4, 5, 6

Enable the Eraser for the compatible local mask (e.g. Brush) you want to edit or rework. If no compatible local mask is found in the image, the eraser appears with a red circle barred, so if you see this, all is fine and there is just no local adjustments area to make a change into.