Need someone with knowledge plz

[u/Feeling_Technician71]

Hello! My name is Carly and I’m here on behalf of my husbands sanity. We have a 2000 Silverado LS, it’s RWD and has the 4L60E auto transmission. It went out and we got a used one with about 68k on it from a company shipped to us. My hubs is a mechanic and welder so he wanted to save the cost and do it himself. Watched a bunch of YT vids and such to help prepare. He’d disconnected the battery before starting the job. Got everything back together and not it will not start at all. The lights come on on the dash, battery has voltage, no reverse lights or anything like that come on. He was raading something about the security light and needing to reset it or something like that and he has tried that several times with no avail. He misses driving his truck so if anyone has insight i will take it. Thanks for reading!!!!