I replaced transmission and now car dont start

On the outside of the transmission or inside of it . How do you know it needs replacing ? Did you test or guessing ? Vehicle won't start in park but moving to neutral it does start ?
Which transmission does it have ?
Automatic Transaxle - 4T45-E
Automatic Transmission - 6T70/6T75

have transmission serviced dont keep trying to drive --if you lucky blew a seal or gasket problem ---if you try to drive it with out fluid can destroy transmission and only fix is to replace it

it could be that your transmission linkage cable between your cabin gear select/shifter and your transmission has become disconnected.

another cause of sticking in a gear would be a torque converter (basically, the "clutch" of an auto transmission) gone bad. first step to check on the transmission physical health is to check the tranny fluid. it should be a very bright, transparent red- free of smell. if it is crimson, dark, or opaque, and/or smells almost "fishy", change the fluid immediately, and dont expect the transmission to hold out for too much longer, if the fluid has that smell and/or is full of metallic specs.

if you have a lot of miles on the vehicle, you may want to have a shop change the automatic transmission fluid anyway, and check for any metal shavings in the drained fluid.

The first mistake never ever flushes transmission!!!

only starting in neutral sounds like the neutral safety switch on the transmission

Hi and welcome to FixYa!
You don't need to replace the whole transmission. I suspect that you have a faulty torque converter. If the torque converter is faulty the shifting of the car becomes sluggish. Torque converter and transmission fluid replacement is surely necessary.
Hope you find this information helpful enough and thank you for using Fixya! Have a good one!

Possible causes Faulty Transmission Control Relay Transmission Control Relay harness is open or shorted Transmission Control Relay circuit poor electrical connection Faulty Transmission Solenoid/Pressure Switch Assembly

Read more: https://www.engine-codes.com/p0888_dodge.html Transmission No Shift Case Study P0888

the transmission control module is messing up....needs replaced

I would give it a tune up spark plugs etc before spending alot of money. most transmission shops do free diagnostics checks.

Unless you are an expert at rebuilding transmissions this is a job you might want to leave to a professional. Once you start to take apart a tranny the odd are when you are done you will have a lot of parts that you dont remember where they went.