I just put a used transmission that was working fine in the vehicle it came from, in my truck and it wont shift and speedometer doesn’t work. It doesn’t slip, just wont shift
First thing to do would be to confirm the fluid level is good. Next check to make sure the electrical connections are good. Look for pinched wires, damaged wiring harness. I have seen the wires pinched between the bell housing when installed. Check shifter adjustment and linkage. If the selector is in 2nd instead of Drive it will move but act like it doesn’t want to shift. Double Check your work. Like you said, it worked fine before you touched it.
Now with that being said, if the transmission you just put in, and know is fine, is now acting like the old one you took out? Then you may have some wiring issues or an issue with the VSS – Vehicle speed sensor sending the right signal to the PCM or a faulty PCM.