I can't see any particular car parts on that sculpture, which might explain it being denied. It looks like a figurative sculpture using iron pipes,... did you try listing it in the Figurative Sculptures category?
I don't think any new sculpture categories are necessary. My understanding is that it is the Subject and Appearance of the sculpture that defines what category it goes into, not the Materials used. Otherwise we would need categories for "Iron Sculptures, Bronze Sculptures, Tin Sculptures, Fibreglass sculptures, Nuts and Bolts Sculptures etc, the list is almost endless.
A Figurative Sculpture can be made of anything (including junk bits and pieces), so long as it is clearly derived from real object sources and really resembles a figure of a human or animal etc. However, Abstract Sculptures are generally attractively thrown-together shapes that represent something but don't really look like anything or maybe vaguely look like something, they break away from traditional representation of physical objects, you have to use your imagination to interpret the art work. Your iron pipe Sculpture obviously looks like a person, and should be accepted as a Figurative Sculpture.
Perhaps I should add, obviously "Car Part Sculptures" a specialty category, have to be made of visible car parts...no matter what the subject (human, animal, vegetable, whatever) of the sculpture is.