There is a switch that goes on the shifter rod where it goes Into the tranny. It is supposed to prevent starting in anything but P and N. you can loosen it and rotate it for adjustment. Your backup lights may not work either. You put it in R with the key on, but not started and rotate until it lights the reverse lights. This may adjust it enough. I had my son attempt to start the car while in P and I rotated the switch until it started without having push on the shifter. It doesn't take much of an adjustment. If alone you can just move it a little until it starts correctly, then tighten.
chevy runs deep, Don't step in chevy!