Q: i just changed transmission but i can not turn on engine......

Hello. If you replaced your transmission and the engine is not starting the first thing I would do is to try to make sure the engine is rotating. You can do this by putting a breaker bar onto the crankshaft and turning it clockwise. If the engine does rotate when turning it likely means that something is not installed correctly and causing the engine and transmission to seize up. You will have to drop the transmission again to double check the proper installation of things such as the torque converter. If the engine did turn when your rotated it clockwise I suggest to look for other things such as electrical connectors being unplugged or blown fuses. Ensure the battery is good and make sure the power is able to reach the vehicle’s electrical system. It will also likely be beneficial to plug a scan tool to the vehicle’s computer to see if there any codes present. Both transmission and engine fault codes should show up during the scan tool testing if there are any present.