Hi all, this is my first post so I apologise if it isn't written correctly or how you're used

I'm having some trouble with solidifying a surface model; I have designed some bodywork for my universities SAE Formula Student car, and I am wanting to separate the Body and sidepod and save them two models with 2mm thickness.

I have been able to Knit the Sidepod, Thicken it, Split and save it as separate part which I am pleased with however when I try to do this with the Body I keep getting the error message 'Failed to offset or face could not be deleted'.

Originally I thought this may have been due to some gaps between surfaces in my model so I tried to fix that, and saved the model as a .step file.
Upon reopening the .step file I was able to run a diagnosis and heal all of these gaps and create one complete surface... This still didnt allow me to thicken the surface and I got the same error message, even with a smaller thickening size 0.1mm etc.

If  anyone could help at all i'd be very greatful! I have attached the solidworks model for anyones reference. Also once I have done this does anyone know how I would completely solidify these models? (Filled volumes)

P.S The model isn't completely finished yet I just wanted to be able to thicken and seperate these parts so that when it is completed I can cut it into sections ready for CNC milling.