Maybe you have a selection that only encompasses the first layer(*). Id Select>None is not grayed out, you have a selection, so just remove it.
(*) To be altered, a pixel has to be 1) in the active layer and 2) covered by the selection mask, if there is one.
(*) To be altered, a pixel has to be 1) in the active layer and 2) covered by the selection mask, if there is one.