The most common cause of this issue is a faulty blower motor resistor. The blower motor resistor is responsible for controlling the speed of the blower motor, which in turn controls the flow of air through the vents. When the blower motor resistor fails, it can cause the blower motor to run only on high speed.
Other possible causes of this issue include a faulty blower motor, a faulty climate control module, or a wiring issue. If you have checked the blower motor resistor and it is not the cause, you will need to have the vehicle inspected by a qualified technician to determine the exact cause of the problem.
