How to edit the stackup design for the single layer board?

Hi...
I had worked two to four layers, currently the single layer pcb board is been preferred by customer. so i want to create an single layer board stackup means, how can i enable he single layer mode in the X section.

Whenever i am tying to get enable the single layer means it's not get enabling on the X section option.So anyone provide me an solution for this.

Thanks & Regards

Dhamodharann