it looks like you're under the child not the parent... that error i notice it also under the parent when any child is added...
I`m pretty sure I`m under the Parent - I`ve tried both wats to get there (see attached)
-selecting the actual parent from the products list
-slecting the parent from inside the child (as you described)
both techniques bring me to the same page - with the same results
[attachment cleanup by admin]