I bought an 00 jimmy with a bad tranmission but it ran. I replaced the transmission and now it won't start. No fuel pressure and I think it has to do with security system. What to Do?

If it cranks it is not the security system, see if it starts spraying carb cleaner into the intake.

It starts with starter fluid.

I would focus on the proper fuel pressure, at least 60 psi.

Sounds like your fuel pump. CAn you hear the fuel pump when you turn the the ignition to the accessory position. Might take another person to help you while you are under the vehicle have your helper turn the key.

The fuel pump will not be energized in the accessory position. When the ignition is turned to the RUN position, the pump should run for 2 to 3 seconds and then shut off. The pump should run continuously in the CRANK position or any time the engine is running.