audiex,
Need more details of your system and how you have your AIO connected.
I use an AIO but do not connect it as per the AIO manufacturers recommendations, I connect both radiator fans directly to the motherboard CPU and CPU_OP fan headers and control fan speeds in the motherboard bios

If you do not have a fan connected to the CPU fan header your bios will report the error you described. You can either connect a fan to the CPU fan header or disable the fan reporting in the bios