2011 Suburban Rear Hatch Won't Open

When you say manually, are you saying you are using the access hole to manually open the gate?
If not, there should be an access hole on the inside of the gate. On my last truck, it was round, about 2", near the bottom center of the gate. The plastic cover should pop off.

I know there is no key hole, I'm inside the truck, at the bottom of the gate. About where you might expect to find the key hole on the outside.