Swapped out motor and starter and the car won’t turn on

Hi, sorry you're having this concern. I have to ask - is this a repair shop asking? Beyond that, I have questions about the motor application. Was it used? Is it an exact replacement for the one that came out -- not "close" but exact? Did you swap wiring harnesses to restore the original harness and hardware with the replacement engine or did you use what came with the engine? There is zero reason to suspect an ECM issue -- except the fact that the ECM may not be compatible with the engine installed. This creates a load of issues that may not be easy resolvable. Is the starter getting a signal? Are you ensuring the keys are properly programmed? So many questions and variables - perhaps ask a local shop near you that may have some ideas also. However, review the first part of my answer first. Good luck.