If your vehicle is showing cannot switch between defrost, floor, and dash vents, here are the most likely causes and the parts you'll need to fix each one.
A small electric motor (actuator) that physically moves the flaps inside the dash to direct airflow has failed or stripped its internal plastic gears. This is the most common cause on modern vehicles.
On many older vehicles (and some heavy-duty trucks), the vent doors are operated by engine vacuum. A cracked vacuum hose, failed check valve, or disconnected line will cause the system to default to the defrost vents for safety.
The climate control panel on the dashboard has failed internally and is no longer sending the correct electrical signals or vacuum routing to the actuators.
If the battery was recently disconnected, replaced, or dashboard work was performed, the HVAC actuators may have lost their calibration limits or a wiring harness was left unplugged.
The physical plastic flap (mode door) or the linkage connecting it to the actuator inside the dashboard has snapped. This prevents air redirection even if the actuator motor is working perfectly.