For what year,model? Sounds like an automatic transmission code. Either that sensor is not plugged in,the sensor is dead,or there is a wiring problem with the sensor.
"The computer wants to know if the transmission is working properly. So it reads the speed of the input shaft and compares it to the speed of the output shaft. It knows which gear you're in because it commanded the transmission to shift into that gear. So, with the input speed, the gear selection, and the output speed, the computer can calculate how efficiently the transmission is working. If the speed is off, it knows that something is slipping inside the transmission. It can then try to boost the fluid pressure to compensate. If that doesn't work, it sets a different trouble code.
The P0717 code means that the computer isn't getting any signal from the sensor. Either the sensor is dead or there's a circuit issue (bad wiring, etc)."