DIAGNOSIS That red light means a power supply fault. The most likely cause is a loose connection where the power cable plugs into the machine's base.
FIX Unplug the machine from the wall. Pull the circular power cable connector straight out of the socket on the back of the purifier's base. Check the pins inside the connector and the socket for any debris or damage, then push it firmly back in until it clicks and sits flush. Plug it back into the wall. The red light should go off and the machine should power up normally.
IF THAT DIDN'T WORK Try a different power outlet with nothing else plugged in. If the red light persists, the issue is likely with the power cable itself or an internal fault. You'll need to try a different Dyson power cable (the one from a Dyson vacuum often works) or contact support.
HEADS UP Never force the connector or try to wiggle it in—it's keyed and only fits one way. If it's not going in smoothly, check the alignment.