Unable to thicken surface

Hi,

I am quite new to Solidworks surfacing, and I am trying to thicken the surface in order to put it through Ansys. I have knitted all the separate components together but the tail seems to be the issue and I can't figure out why. It is a lofted surface with some surface fill and when I try to thicken it I get the following prompt: "Failed to offset or face could not be deleted".

Does anyone know how this can be fixed? Or how I can recreate the surface so that it will work?

Any advice would be greatly appreciated.

Thanks!

SolidworksSurfacing