I see the source of the problem. Normally the CPU cooler (fan, for example) is plugged into the CPU_FAN header which does three jobs. It provides power to the fan, it controls the fan speed, and it monitors the speed signal sent back to it from the fan for possible FAILURE. If there is NO fan speed signal, it understands the fan has failed, sends out an immediate prominent warning, and often will shut down the whole system WITHOUT waiting for the temp sensor inside the CPU chip to detect overheating. That system then may refuse to allow you to boot up with NO fan speed signal at the CPU_FAN header.
Most AIO liquid-cooling systems have a cable from the pump unit that you plug into the CPU_FAN header so that the PUMP speed signal can be sent back there, and that keeps the CPU_FAN header's failure detection system happy. However, that Aorus system you have, according to its manual, has NO such cable. It says nothing at all about plugging anything into the CPU_FAN header. And it does say clearly that the rad fans should be plugged into the outputs from the pump unit so that the Aorus software tool can manage those fans' speeds. The result is exactly as you describe: the CPU_FAN header, with nothing plugged into it, detects a CPU cooling system failure, puts out an alarm, and refuses to allow you to proceed.
You have discovered that you can prevent this problem by connecting another fan to that header. But you still have a different issue. The Aorus software tool should show you the speeds of the rad fans (maybe only one fan speed) and of the pump. But you say this is not happening. The method that software uses for this AND to control the system AND to monitor itself for failures is the cable that you are supposed to connect from the Pump unit to a mobo USB2 header. That is how the software communicates with the entire AIO system. So, check on that cable and ensure it is connected properly. It MAY be that, once that is working and the software CAN verify proper function of the pump and fans, it also will alter the CPU_FAN header failure detection system to deactivate it. THEN, if you remove the case fan you have temporarily plugged into the CPU_FAN header and plug it back into its normal SYS_FAN or CHA_FAN header, the system will still be happy and NOT believe it has a failed CPU cooler.
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