Failed speed sensors, leaking cooler lines, or a leaking solenoid pack.
I had one. I had to have the transmission rebuilt. Nightmare!!
The vast majority of cars in the US have automatic transmission. This is not true in many other countries.
What year and which trans?
Take it to a dealer or fully equipped service shop that can access the trouble codes stored in the vehicle's on-baord computer and they may be able to tell you exactly what the problem is.
A transmission that shifts through gears after moving lever to "d" without driver doing anything no clutch pedal