There are ~5 surfaces in your design, not sure which surface you are trying to thicken, which direction or by how much.
I was able to thicken the back triangle surface bu 1mm towards the inside, the rest failed 1mm.
If you could provide some more details on what you are trying to do, that would help.
Hello Ahsan,.. as Andrew Schutte said,.. what needs Thickening?
I could guess the 1mm wall thickness used earlier and the SurfaceKnits need to be thickened?:
so,.. for the Fillet57 and Mirror6 .. I would suggest doing the thicken earlier... add Fill/blend later.
and, for the Fillet 78 and Fillet 89, I would suggest not using Ruled Surfaces.. just offset "0" and thicken 1mm and add fillets
You'll find a few problems thickening a complex shape like that. Your best bet is to offset the surface(s) you are trying to thicken, then loft from one to the other to create the solid, using guide curves where necessary. I've made a few helmets in the past and thickening tends not to give you the best geometry anyway.
Good luck and the pictures look good.