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

This information applies to all versions

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.