My Transmission Won't Engage

I have an issue now, that I replaced the engine in a 1991 Ford Explorer but the thing is that after I replaced the engine I noticed that my transmission won’t engage at any shift position nor feeling any engage or change of RPM while engaging, it’s a mystery for me, any help anybody?

Did you remember to bolt the torque converter to the flex plate on the back of the engine?? Did you lose any ATF during the engine exchange??