Surface Thicken wont work

Hi guys,

Im trying to thicken a surface after creating surface offset, but the error Im getting is "face which failed check". 

The surface Im trying to thicken is the frame around the cockpit windows.

Anyone any ideas what the problem is and the solution for it?

Ive attached the file and screenshots for your viewing.

Thanks

Lee

The surface Im trying to thicken is the frame around the cockpit windows.

SolidworksGeneral