Blazernick

Hello all, I am having issues with my blazers Ecu, first off I bought another blazer to rebuild mine from a previous accident so I put my 1998 blazer back together hooked up the Ecu and nothing. The only things on the dash that work is the voltage gauge and the airbag light, if I leave the key in the ignition for 30 seconds the secuirity light comes up solid and will not go away I have waited 20 mins and it never went away, also it turns over but no fuel or spark what could the issue be I even put the Ecu off the other blazer on (yes I know it must be programmed to vin) but it didn't to a thing thought it would at least show something, the headlights work, interior lights, dome lights and radio even the switches I am really confused on why it's not detecting the Ecu thanks for viewing I would appreciate some input thanks.