Just put transmission in car its taking off in 2nd gear not first n o/d light flashing what's the problem

Fix what trying to see if anyone may know what the problem is

that is a very complicated transmission.if anyone gives you an answer here,they would most likely be guessing!i have the same transmission in my truck.it's a 4R70W transmission model,unless it has the (Police interceptor)then it's a 4R75W model.i had problems with mine 6 years ago,it was over my head.local family owned trusted transmission shop had to rebuild it.i took your question as you just having it rebuilt.that's why i suggested calling the transmission shop back.sorry!