transmission does not shift into reverse or neutral?

This is a valve body problem one of the valves is stuck. Here is how to remove the valve body so you can get a rebuilt one.

You will need to drop the pan and drain the fluid and refill. This guide should help walk you through the job:

https://www.2carpros.com/articles/how-to-service-an-automatic-transmission

REMOVAL
1. Raise and support vehicle, drain transmission fluid, then remove transmission pan, gasket, and filter.
2. Remove detent spring attaching bolt, then the spring.
3. Remove valve body to case attaching bolts, then the valve body.

INSTALLATION
1. Reverse procedure to install.
2. Use suitable guide pins to align valve body to case.

Please let us know if you need anything else to get the problem fixed.