Have you at least checked the transmission fluid? Has the transmission ever been serviced? Is the check engine light on?
You’re going to need to take it to a transmission shop and have it put on a scanner. You don’t need a dealer, do not go to just any mechanic, and do not go to a national chain transmission shop.
Just ask around for the best local, owner-operated transmission shop and bring them the car.
There are any number of reasons this can happen so guessing won’t do much good.