Layer mask is editable as soon as you create it. When you go back to edit it it cannot be selected.
I've found a quick solution to this, but not why this is happening. This happened to me, I couldn't click on the actual mask itself. No way to select it. Nothing in google came up. I then double clicked the layer which popped up the layer properties. Then I clicked on the little white flag with the black circle in the middle (layer mask symbol). It selected the mask. If you hover over it, it'll say "Select the layer mask" and you can select the mask that way. This is the only file I have that is doing this too. The rest work fine when I click the mask. Don't know why this happened, and it's so annoying, but I wanted to make this post to let you know how I figured out how to get around it.