hello judy.wang,
you should use fan with 5V regulator,
please also make sure you had proper pin connections.
for example,
Pin-1 -> GND
Pin-2 -> 5V
Pin-3 -> Tach
Pin-4 -> PWM
the RPM measurement should works with proper fan device and also with proper pin connections,
it can be read by enabling tach from SW commands,
for example,
# echo 1 > /sys/class/hwmon/hwmon1/tach_enable
# cat /sys/class/hwmon/hwmon1/rpm_measured