my subject mask wont save

Sounds like you made a common mistake, by thinking that the mask overlay is an edit. That black background is not an edit, it is one of the view options that you can use to see what is masked and what is not masked. The edits are applied to the masked area only, and are made by changing the sliders. If you do not change any sliders, and it sounds like you did not, then your mask is not going to do anything. Getting a truely black background is not that easy at all. Try setting the exposure slider of the mask all the way to the left (-5.0).