I cannot add a layer where I need to

I am trying to put the 'left hands' layer into the left arm portion of my work. I need it that way to have the hands interchangeable in Character Animate. It worked fine when I did it with the right arm but the left arm shows up with a black circle with  a line through it telling me that it cannot be done. I can find no reason for this restriction. I have unlocked all layers and it is the only one with other layers within it that wont let me add to it. Help!