Adding via inside BGA cell

Hi all,

I am having troubles with adding a via inside a BGA cell. Previously I was able to add. But now it does not allow me to do so. Following error message was given by xPCB VX.1.

I am new to this software. So I think I may have enable something unknowingly.

Add 1-4 via failed for net GND when changing to layer 1

Placement Outline Bottom ; Bot; loc: (8.61, 5.17)(23.39, 61.94)

Thank you