Hello and welcome to JustAnswer!
If you aren't able to switch to defrost there are a few things that could be wrong.
It could be a broken door. There are doors in the heater box which control airflow, the mode or defrost door may be broken or bound up and preventing air from coming from defrost.
It could be a bad actuator. the doors are controlled by electric actuators, if an actuator has stripped or isn't functioning electrically then it will prevent the door from moving.
It could be the control head on the dash or a wiring issue between the control head and actuator.
A list of what it could be is a joke. I realise it could be the control unit the vet stepper motor or the door jammed. The me what I didn't know.
I don't know what kind of help you're looking for without communication. What kind of help are you needing?