unable to read vehicle (1) message

2002 chevy silverado bought in summit 2 years ago still can't get it to work i have had several em with mike l and no results i would really like to get it working please any help would be great
i have found online many people having same problem as me plug it in and does nothing just reads unable to read vehicle (1) message

[06MonteSS]

have your key turned to ON position?

have everything inside turned off - radio, lights, fan/blower, etc?

anything aftermarket installed in the truck? if so, you need to disconnect/pull the fuses for anything aftermarket.. like aftermarket stereo, subs, amps, nav, remote start, alarm, trailer brake/light setup, etc.. anything that did not come from the factory needs to be disabled/pull the fuses for it...