Won’t back up stock time

Just got a new to me trinity. Person I got it from used it with no issues. When I plug it in it says cannot connect to pcm and tries a couple times but nothing. Anyone else have this issues? 2004 duramax LB7

[06MonteSS]

Anything aftermarket installed in the vehicle? i.e did not come from the factory?

You need to make sure anything aftermarket is disconnected/pull the fuse for - anything like aftermarket stereo, subs, amps, nav, alarm, remote start, seat heaters, trailer brake/light setup, etc.. anything like that, their wiring harnesses need to be disconnected/pull the fuses for it before you try to tune anything...

make sure truck battery is fully charged...

also make sure everything is turned off inside the truck.. lights, radio, heat/fan/blower, do not operate power windows, put your parking brake on, etc...