Question ProBuilderize is not coming out as expected

I downloaded some car models on the internet and I wanted to modify some parts of the car with ProBuilder, but when I do ProBuilderize with the Import Quads and Import Smoothing option activated, some faces disappear and the object is perforated, if I deactivate the Import Smoothing option the object has a strange color and the material remains the same without changes, does anyone know how to fix this? I wanted to modify the object directly within Unity so as not to need external modeling programs.