Asus motherboards are notorius for not being able to report fan speed below a certain RPM. Both of my crosshairs and a few intel boards i've built have caused a false positive on the fan error. You can disable "pause on fan error". It'll still show the error but just keep booting.
You can also adjust the fan speed curve in the BIOS to make the fan spin faster at cooler temps, but you increase noise.
Nothing to be worried about.