replacing ecm and cant get security light to show on message center to reteach on 2000 Chevrolet Silverado 1500

tried the pk111 procedure several times with no results, wont start. put old ecm in starts ok. whats the trick. truch wont shift out of second gear, reset ecm and it will drive and shift good until shut off and restarted then malfunctions. after malfunction shows po758 code, replaced everything in trans. unplug old ecm, reset and shifts fine again until shut off, then all over again. bought rebuilt ecm, cant get security light on to reteach

Does that ECM have the program in it? You may read the instrutions supplied with it or check with store you purchaced the ECM from. It may require to be programed with latest software first.
comes programed and flashed, sorry to leave out that important info.